Demo – Anatomy

The liver is the largest gland in the body and, after the skin, the largest single organ. It weighs approximately 1500 g and accounts for approximately 2.5% of adult body weight. In a mature fetus when it serves as a hematopoietic organ it is proportionately twice as large (5% of body weight).

Except for fat, all nutrients absorbed from the gastrointestinal tract are initially conveyed to the liver by the portal venous system. In addition to its many metabolic activities, the liver stores glycogen and secretes bile, a yellow-brown or green fluid that aids in the emulsification of fat.

Bile passes from the liver via the biliary ducts—right and left hepatic ducts—that join to form the common hepatic duct, which unites with the cystic duct to form the (common) bile duct. The liver produces bile continuously; however, between meals it accumulates and is stored in the gallbladder, which also concentrates the bile by absorbing water and salts. When food arrives in the duodenum, the gallbladder sends concentrated bile through the biliary ducts to the duodenum.

SURFACE ANATOMY, SURFACES, PERITONEAL REFLECTIONS, AND RELATIONSHIPS OF LIVER

The liver lies mainly in the right upper quadrant of the abdomen, where it is protected by the thoracic (rib) cage and the diaphragm. The normal liver lies deep to ribs 7–11 on the right side and crosses the midline toward the left nipple. The liver occupies most of the right hypochondrium and upper epigastrium and extends into the left hypochondrium. The liver moves with the excursions of the diaphragm and is located more inferiorly when one is erect because of gravity. This mobility facilitates palpation.

The liver has a convex diaphragmatic surface (anterior, superior, and some posterior) and a relatively flat or even concave visceral surface (postero-inferior), which are separated anteriorly by its sharp inferior border that follows the right costal margin. inferior to the diaphragm. The diaphragmatic surface of the liver is smooth, and dome shaped, where it is related to the concavity of the inferior surface of the diaphragm, which separates it from the pleurae, lungs, pericardium, and heart. Subphrenic recesses—superior extensions of the peritoneal cavity (greater sac)—exist between diaphragm and the anterior and superior aspects of the diaphragmatic surface of the liver. The subphrenic recesses are separated into right and left recesses by the falciform ligament, which extends between the liver and the anterior abdominal wall. The portion of the supracolic compartment of the peritoneal cavity immediately inferior to the liver is the subhepatic space.

The hepatorenal recess (hepatorenal pouch; Morison pouch) is the posterosuperior extension of the subhepatic space, lying between the right part of the visceral surface of the liver and the right kidney and suprarenal gland. The hepatorenal recess is a gravity-dependent part of the peritoneal cavity in the supine position; fluid draining from the omental bursa flows into this recess. The hepatorenal recess communicates anteriorly with the right subphrenic recess. Recall that normally all recesses of the peritoneal cavity are potential spaces only, containing just enough peritoneal fluid to lubricate the adjacent peritoneal membranes.

The diaphragmatic surface of the liver is covered with visceral peritoneum, except posteriorly in the bare area of the liver, where it lies in direct contact with the diaphragm. The bare area is demarcated by the reflection of peritoneum from the diaphragm to it as the anterior (upper) and posterior (lower) layers of the coronary ligament. These layers meet on the right to form the right triangular ligament and diverge toward the left to enclose the triangular bare area. The anterior layer of the coronary ligament is continuous on the left with the right layer of the falciform ligament, and the posterior layer is continuous with the right layer of the lesser omentum. Near the apex (the left extremity) of the wedge-shaped liver, the anterior and posterior layers of the left part of the coronary ligament meet to form the left triangular ligament. The IVC traverses a deep groove for the vena cava within the bare area of the liver.

The visceral surface of the liver is also covered with visceral peritoneum, except in the fossa for the gallbladder and the porta hepatis—a transverse fissure where the vessels (hepatic portal vein, hepatic artery, and lymphatic vessels), the hepatic nerve plexus, and hepatic ducts that supply and drain the liver enter and leave it. In contrast to the smooth diaphragmatic surface, the visceral surface bears multiple fissures and impressions from contact with other organs.

Two sagittally oriented fissures, linked centrally by the transverse porta hepatis, form the letter H on the visceral surface. The right sagittal fissure is the continuous groove formed anteriorly by the fossa for the gallbladder and posteriorly by the groove for the vena cava. The umbilical (left sagittal) fissure is the continuous groove formed anteriorly by the fissure for the round ligament and posteriorly by the fissure for the ligamentum venosum. The round ligament of the liver (L. ligamentum teres hepatis) is the fibrous remnant of the umbilical vein, which carried well-oxygenated and nutrient-rich blood from the placenta to the fetus. The round ligament and small para-umbilical veins course in the free edge of the falciform ligament. The ligamentum venosum is the fibrous remnant of the fetal ductus venosus, which shunted blood from the umbilical vein to the IVC, short-circuiting the liver.

The lesser omentum, enclosing the portal triad (bile duct, hepatic artery, and hepatic portal vein) passes from the liver to the lesser curvature of the stomach and the first 2 cm of the superior part of the duodenum. The thick, free edge of the lesser omentum extends between the porta hepatis and the duodenum (the hepatoduodenal ligament) and encloses the structures that pass through the porta hepatis. The sheet-like remainder of the lesser omentum, the hepatogastric ligament, extends between the groove for the ligamentum venosum and the lesser curvature of the stomach.

In addition to the fissures, impressions on (areas of) the visceral surface reflect the liver's relationship to the:

• Right side of the anterior aspect of the stomach (gastric and pyloric areas).
• Superior part of the duodenum (duodenal area).
• Lesser omentum (extends into the fissure for the ligamentum venosum).
• Gallbladder (fossa for gallbladder).
• Right colic flexure and right transverse colon (colic area).
• Right kidney and suprarenal gland (renal and suprarenal areas)

ANATOMICAL LOBES OF LIVER

Externally, the liver is divided into two anatomical lobes and two accessory lobes by the reflections of peritoneum from its surface, the fissures formed in relation to those reflections and the vessels serving the liver and the gallbladder. These superficial “lobes” are not true lobes as the term is generally used in relation to glands and are only secondarily related to the liver's internal architecture. The essentially midline plane defined by the attachment of the falciform ligament, and the left sagittal fissure separates a large right lobe from a much smaller left lobe. On the slanted visceral surface, the right and left sagittal fissures course on each side of—and the transverse porta hepatis separates—two accessory lobes (parts of the anatomic right lobe): the quadrate lobe anteriorly and inferiorly and the caudate lobe posteriorly and superiorly. The caudate lobe was so-named not because it is caudal in position (it is not) but because it often gives rise to a “tail” in the form of an elongated papillary process. A caudate process extends to the right, between the IVC and the porta hepatis, connecting the caudate and right lobes.

FUNCTIONAL SUBDIVISION OF LIVER

Although not distinctly demarcated internally, where the parenchyma appears continuous, the liver has functionally independent right and left livers (parts or portal lobes) that are much more equal in size than the anatomical lobes; however, the right liver is still somewhat larger. Each part receives its own primary branch of the hepatic artery and hepatic portal vein and is drained by its own hepatic duct. The caudate lobe may in fact be considered a third liver; its vascularization is independent of the bifurcation of the portal triad (it receives vessels from both bundles) and is drained by one or two small hepatic veins, which enter directly into the IVC distal to the main hepatic veins. The liver can be further subdivided into four divisions and then into eight surgically resectable hepatic segments, each served independently by a secondary or tertiary branch of the portal triad, respectively.

Hepatic (Surgical) Segments of Liver. Except for the caudate lobe (segment I), the liver is divided into right and left livers based on the primary (1°) division of the portal triad into right and left branches, the plane between the right and the left livers being the main portal fissure in which the middle hepatic vein lies. On the visceral surface, this plane is demarcated by the right sagittal fissure. The plane is demarcated on the diaphragmatic surface by extrapolating an imaginary line—the Cantlie line (Cantlie, 1898)—from the notch for the fundus of the gallbladder to the IVC. The right and left livers are subdivided vertically into medial and lateral divisions by the right portal and umbilical fissures, in which the right and left hepatic veins lie. The right portal fissure has no external demarcation. Each of the four divisions receives a secondary (2°) branch of the portal triad. A transverse hepatic plane at the level of the horizontal parts of the right and left branches of the portal triad subdivides three of the four divisions (all but the left medial division), creating six hepatic segments, each receiving tertiary branches of the triad. The left medial division is also counted as a hepatic segment, so that the main part of the liver has seven segments (segments II–VIII, numbered clockwise), which have also been given a descriptive name. The caudate lobe (segment I, bringing the total number of segments to eight) is supplied by branches of both divisions and is drained by its own minor hepatic veins.

While the pattern of segmentation described here is the most common pattern, the segments vary considerably in size and shape as a result of individual variation in the branching of the hepatic and portal vessels.

BLOOD VESSELS OF LIVER

The liver, like the lungs, has a dual blood supply (afferent vessels): a dominant venous source and a lesser arterial one. The hepatic portal vein brings 75–80% of the blood to the liver. Portal blood, containing about 40% more oxygen than blood returning to the heart from the systemic circuit, sustains the liver parenchyma (liver cells or hepatocytes). The hepatic portal vein carries virtually all of the nutrients absorbed by the alimentary tract to the sinusoids of the liver. The exception is lipids, which are absorbed into and bypass the liver via the lymphatic system. Arterial blood from the hepatic artery, accounting for only 20–25% of blood received by the liver, is distributed initially to non-parenchymal structures, particularly the intrahepatic bile ducts.

The hepatic portal vein, a short, wide vein, is formed by the superior mesenteric and splenic veins posterior to the neck of the pancreas. It ascends anterior to the IVC as part of the portal triad in the hepatoduodenal ligament. The hepatic artery, a branch of the celiac trunk, may be divided into the common hepatic artery, from the celiac trunk to the origin of the gastroduodenal artery, and the hepatic artery proper, from the origin of the gastroduodenal artery to the bifurcation of the hepatic artery. At or close to the porta hepatis, the hepatic artery and hepatic portal vein terminate by dividing into right and left branches; these primary branches supply the right and left livers, respectively. Within the right and left livers, the simultaneous secondary branchings of the hepatic portal vein and hepatic artery supply the medial and lateral divisions of the right and left liver, with three of the four secondary branches undergoing further (tertiary) branchings to supply independently seven of the eight hepatic segments.

Between the divisions are the right, intermediate (middle), and left hepatic veins, which are intersegmental in their distribution and function, draining parts of adjacent segments. The hepatic veins, formed by the union of collecting veins that in turn drain the central veins of the hepatic parenchyma, open into the IVC just inferior to the diaphragm. The attachment of these veins to the IVC helps hold the liver in position.

LYMPHATIC DRAINAGE AND INNERVATION OF LIVER

The liver is a major lymph-producing organ. Between one quarter and one half of the lymph entering the thoracic duct comes from the liver.

The lymphatic vessels of the liver occur as superficial lymphatics in the subperitoneal fibrous capsule of the liver (Glisson capsule), which forms its outer surface, and as deep lymphatics in the connective tissue, which accompany the ramifications of the portal triad and hepatic veins. Most lymph is formed in the perisinusoidal spaces (of Disse) and drains to the deep lymphatics in the surrounding intralobular portal triads.

Superficial lymphatics from the anterior aspects of the diaphragmatic and visceral surfaces of the liver, and deep lymphatic vessels accompanying the portal triads, converge toward the porta hepatis. The superficial lymphatics drain to the hepatic lymph nodes scattered along the hepatic vessels and ducts in the lesser omentum. Efferent lymphatic vessels from the hepatic nodes drain into celiac lymph nodes, which in turn drain into the cisterna chyli (chyle cistern), a dilated sac at the inferior end of the thoracic duct.

Superficial lymphatics from the posterior aspects of the diaphragmatic and visceral surfaces of the liver drain toward the bare area of the liver. Here they drain into phrenic lymph nodes or join deep lymphatics that have accompanied the hepatic veins converging on the IVC and pass with this large vein through the diaphragm to drain into the posterior mediastinal lymph nodes. Efferent lymphatic vessels from these nodes join the right lymphatic and thoracic ducts. A few lymphatic vessels follow different routes:

• From the posterior surface of the left lobe of the liver toward the esophageal hiatus of the diaphragm to end in the left gastric lymph nodes.
• From the anterior central diaphragmatic surface along the falciform ligament to the parasternal lymph nodes.
• Along the round ligament of the liver to the umbilicus and lymphatics of the anterior abdominal wall.

The nerves of the liver are derived from the hepatic plexus, the largest derivative of the celiac plexus. The hepatic plexus accompanies the branches of the hepatic artery and hepatic portal vein to the liver. This plexus consists of sympathetic fibers from the celiac plexus and parasympathetic fibers from the anterior and posterior vagal trunks. Nerve fibers accompany the vessels and biliary ducts of the portal triad. Other than vasoconstriction, their function is unclear.

Clinically oriented anatomy 7^th ed.

The lumbar plexus of nerves is formed anterior to the lumbar transverse processes, within the proximal attachment of the psoas major. This nerve network is composed of the anterior rami of L1 through L4 nerves. The following nerves are branches of the lumbar plexus; the three largest are listed first:

• The femoral nerve (L2–L4) emerges from the lateral border of the psoas major and innervates the iliacus and passes deep to the inguinal ligament/iliopubic tract to the anterior thigh, supplying the flexors of the hip and extensors of the knee.

• The obturator nerve (L2–L4) emerges from the medial border of the psoas major and passes into the lesser pelvis, passing inferior to the superior pubic ramus (through the obturator foramen) to the medial thigh, supplying the adductor muscles.

• The lumbosacral trunk (L4, L5) passes over the ala (wing) of the sacrum and descends into the pelvis to participate in the formation of the sacral plexus with the anterior rami of S1–S4 nerves.

• The ilio-inguinal and iliohypogastric nerves (L1) arise from the anterior ramus of L1, entering the abdomen posterior to the medial arcuate ligament and passing inferolaterally, anterior to the quadratus lumborum. They run superior and parallel to the iliac crest, piercing the transversus abdominis near the ASIS. They then pass through the internal and external obliques to supply the abdominal muscles and skin of the inguinal and pubic regions. The division of the L1 anterior ramus may occur as far distally as the ASIS, so that often only one nerve (L1) crosses the posterior abdominal wall instead of two.

• The genitofemoral nerve (L1, L2) pierces the psoas major and runs inferiorly on its anterior surface, deep to the psoas fascia; it divides lateral to the common and external iliac arteries into femoral and genital branches.

• The lateral cutaneous nerve of the thigh, or lateral femoral cutaneous nerve (L2, L3), runs inferolaterally on the iliacus and enters the thigh deep to the inguinal ligament/iliopubic tract, just medial to the ASIS; it supplies skin on the anterolateral surface of the thigh.

An accessory obturator nerve (L3, L4) is present almost 10% of the time. It parallels the medial border of the psoas, anterior to the obturator nerve, crossing superior to the superior pubic ramus in close proximity to the femoral vein.

Although the larger branches (femoral, obturator, and lumbosacral trunk) are consistent in their placement, variation should be anticipated in the disposition of the smaller branches of the lumbar plexus.

Clinically oriented anatomy 7th ed.

The external intercostal muscles (11 pairs) occupy the intercostal spaces from the tubercles of the ribs posteriorly to the costochondral junctions anteriorly.

Anteriorly, the muscle fibres are replaced by the external intercostal membranes.

These muscles run infero-anteriorly from the rib above to the rib below. Each muscle attaches superiorly to the inferior border of the rib above and inferiorly to the superior border of the rib below. These muscles are continuous inferiorly with the external oblique muscles in the anterolateral abdominal wall. The external intercostals are most active during inspiration.

Clinically oriented anatomy 7th ed.

The urinary bladder, a hollow viscus with strong muscular walls, is characterized by its distensibility. The bladder is a temporary reservoir for urine, and varies in size, shape, position, and relationships according to its content, and the state of neighbouring viscera.

When empty, the adult urinary bladder is located in the lesser pelvis, lying partially superior to and partially posterior to the pubic bones. It is separated from these bones by the potential retropubic space (of Retzius) and lies mostly inferior to the peritoneum, resting on the pubic bones and pubic symphysis anteriorly and the prostate (males) or anterior wall of the vagina (females) posteriorly. The bladder is relatively free within the extraperitoneal subcutaneous fatty tissue, except for its neck, which is held firmly by the lateral ligaments of bladder and the tendinous arch of the pelvic fascia—especially its anterior component, the puboprostatic ligament in males and the pubovesical ligament in females. In females, since the posterior aspect of the bladder rests directly upon the anterior wall of the vagina, the lateral attachment of the vagina to the tendinous arch of the pelvic fascia, the paracolpium, is an indirect but important factor in supporting the urinary bladder.

In infants and young children, the urinary bladder is in the abdomen even when empty. The bladder usually enters the greater pelvis by 6 years of age; however, it is not located entirely within the lesser pelvis until after puberty. An empty bladder in adults lies almost entirely in the lesser pelvis, its superior surface level with the superior margin of the pubic symphysis. As the bladder fills, it enters the greater pelvis as it ascends in the extraperitoneal fatty tissue of the anterior abdominal wall. In some individuals, a full bladder may ascend to the level of the umbilicus.

At the end of micturition (urination), the bladder of a normal adult contains virtually no urine. When empty, the bladder is somewhat tetrahedral and externally has an apex, body, fundus, and neck. The bladder’s four surfaces (superior, two inferolateral, and posterior) are most apparent when viewing an empty, contracted bladder that has been removed from a cadaver, when the bladder appears rather boat shaped.

The apex of the bladder points toward the superior edge of the pubic symphysis when the bladder is empty. The fundus of the bladder is opposite the apex, formed by the somewhat convex posterior wall. The body of the bladder is the major portion of the bladder between the apex and the fundus. The fundus and inferolateral surfaces meet inferiorly at the neck of the bladder.

The bladder bed is formed by the structures that directly contact it. On each side, the pubic bones and fascia covering the levator ani and the superior obturator internus lie in contact with the inferolateral surfaces of the bladder. Only the superior surface is covered by peritoneum. Consequently, in males the fundus is separated from the rectum centrally by only the fascial rectovesical septum and laterally by the seminal glands and ampullae of the ductus deferentes. In females the fundus is directly related to the superior anterior wall of the vagina. The bladder is enveloped by a loose connective tissue visceral fascia.

The walls of the bladder are composed chiefly of the detrusor muscle. Toward the neck of the male bladder, the muscle fibres form the involuntary internal urethral sphincter. This sphincter contracts during ejaculation to prevent retrograde ejaculation (ejaculatory reflux) of semen into the bladder. Some fibres run radially and assist in opening the internal urethral orifice. In males, the muscle fibres in the neck of the bladder are continuous with the fibromuscular tissue of the prostate, whereas in females these fibres are continuous with muscle fibres in the wall of the urethra.

The ureteric orifices and the internal urethral orifice are at the angles of the trigone of the bladder. The ureteric orifices are encircled by loops of detrusor musculature that tighten when the bladder contracts to assist in preventing reflux of urine into the ureter. The uvula of the bladder is a slight elevation of the trigone; it is usually more prominent in older men owing to enlargement of the posterior lobe of the prostate.

Arterial Supply and Venous Drainage of Bladder. The main arteries supplying the bladder are branches of the internal iliac arteries. The superior vesical arteries supply anterosuperior parts of the bladder. In males, the inferior vesical arteries supply the fundus and neck of the bladder. In females, the vaginal arteries replace the inferior vesical arteries and send small branches to posteroinferior parts of the bladder. The obturator and inferior gluteal arteries also supply small branches to the bladder.

The veins draining blood from the bladder correspond to the arteries and are tributaries of the internal iliac veins. In males, the vesical venous plexus is continuous with the prostatic venous plexus, and the combined plexus complex envelops the fundus of the bladder and prostate, the seminal glands, the ductus deferentes, and the inferior ends of the ureters. It also receives blood from the deep dorsal vein of the penis, which drains into the prostatic venous plexus. The vesical venous plexus is the venous network that is most directly associated with the bladder itself. It mainly drains through the inferior vesical veins into the internal iliac veins; however, it may drain through the sacral veins into the internal vertebral venous plexuses. In females, the vesical venous plexus envelops the pelvic part of the urethra and the neck of the bladder, receives blood from the dorsal vein of the clitoris, and communicates with the vaginal or uterovaginal venous plexus.

Innervation of Bladder. Sympathetic fibres are conveyed from inferior thoracic and upper lumbar spinal cord levels to the vesical (pelvic) plexuses primarily through the hypogastric plexuses and nerves, whereas parasympathetic fibres from sacral spinal cord levels are conveyed by the pelvic splanchnic nerves and the inferior hypogastric plexus. The parasympathetic fibres are motor to the detrusor muscle and inhibitory to the internal urethral sphincter of the male bladder. Hence, when visceral afferent fibres are stimulated by stretching, the bladder contracts reflexively, the internal urethral sphincter relaxes (in males), and urine flows into the urethra. With toilet training, we learn to suppress this reflex when we do not wish to void. The sympathetic innervation that stimulates ejaculation simultaneously causes contraction of the internal urethral sphincter, to prevent reflux of semen into the bladder. A sympathetic response at moments other than ejaculation (e.g., self-consciousness when standing at the urinal in front of a waiting line) can cause the internal sphincter to contract, hampering the ability to urinate until parasympathetic inhibition of the sphincter occurs.

Sensory fibres from most of the bladder are visceral; reflex afferents follow the course of the parasympathetic fibres, as do those transmitting pain sensations (such as results from overdistension) from the inferior part of the bladder. The superior surface of the bladder is covered with peritoneum and is therefore superior to the pelvic pain line; thus pain fibres from the superior bladder follow the sympathetic fibres retrogradely to the inferior thoracic and upper lumbar spinal ganglia (T11–L2 or L3).

Clinically oriented anatomy 7^th ed.

HISTOLOGY OF THE RESPIRATORY SYSTEM

The respiratory system consists of an air conducting region (the upper respiratory tract in the head, as well as the larynx, trachea, bronchi, and most bronchioles) and a respiratory region with alveoli.

 Respiratory Epithelium

Most of the nasal cavities and conducting portion of the system is lined with mucosa having ciliated pseudostratified colum-nar epithelium,

This epithelium has five major cell types, all of which contact an unusually thick basement membrane:

• Ciliated columnar cells are the most abundant, each with 250-300 cilia on its apical surface.
• Goblet cells are also numerous and predominate in some areas, with basal nuclei and apical domains filled with granules of mucin glycoproteins.
• Brush cells are a much less numerous, columnar cell type, in which a small apical surface bears sparse, blunt microvilli. Brush cells are chemosen­sory receptors resembling gustatory cells, with similar signal transduction components and synaptic contact with afferent nerve endings on their basal surfaces.
• Small granule cells (or Kulchitsky cells) are difficult to distinguish in routine preparations but possess numer­ous dense core granules 100-300 nm in diameter. Like enteroendocrine cells of the gut, they are part of the diffuse neuroendocrine system (DNES). Like brush cells, they represent only about 3% of the cells in respiratory epithelium.
• Basal cells are mitotically active stem and progenitor cells that give rise to the other epithelial cell types.

(Junqueira Basic Histology Text and Atlas)

EMBRYOLOGY OF THE CARDIOVASCULAR SYSTEM

The cardiovascular system begins to develop at the end of the third week. The primordial heart starts to beat at the beginning of the fourth week. Mesenchymal cells derived from splanchnic mesoderm proliferate and form isolated cell clusters, which soon develop into two heart tubes that join to form the primordial vascular system. Splanchnic mesoderm surrounding the heart tube forms the primordial myocardium. The heart primordium consists of four chambers: the bulbus cordis, ventricle, atrium, and sinus venosus.

The truncus arteriosus (primordium of the ascending aorta and pulmonary trunk) is continuous caudally with the bulbus cordis, which becomes part of the ventricles. As the heart grows, it bends to the right and soon acquires the general external appearance of the adult heart. The heart becomes partitioned into four chambers between the fourth and seventh weeks.

Three systems of paired veins drain into the primordial heart: the vitelline system, which becomes the portal system; the cardinal veins, which form the caval system; and the umbilical veins, which involute after birth.

As the pharyngeal arches form during the fourth and fifth weeks, they are penetrated by pharyngeal arteries that arise from the aortic sac. During the sixth to eight weeks, the pharyngeal arch arteries are transformed into the adult arterial arrangement of the carotid, subclavian, and pulmonary arteries.

The critical period of heart development is from day 20 to day 50 after fertilization. Numerous events occur during cardiac development, and deviation from the normal pattern at any time may produce one or more CHDs. Because partitioning of the primordial heart results from complex cellular and molecular processes, defects of the cardiac septa are relatively common particularly VSDs. Some birth defects result from abnormal transformation of the pharyngeal arch arteries into the adult arterial pattern.

Because the lungs are nonfunctional during prenatal life, the fetal cardiovascular system is structurally designed so that blood is oxygenated in the placenta and most of it bypasses the lungs. The modifications that establish the postnatal circulatory pattern are not abrupt but extend into infancy. Failure of these changes in the circulatory system to occur at birth results in two of the most common congenital anomalies of the heart and great vessels: patent foramen ovale and patent ductus arteriosus.

The lymphatic system begins to develop late in the sixth week in close association with the venous system. Six primary lymph sacs develop, which later become interconnected by lymphatic vessels. Lymph nodes develop along the network of lymphatic vessels; lymph nodules do not appear until just before or after birth.

(Keith Moore Embryology)

EAR

The ear—the organ of hearing and equilibrium (balance)—is divided into the external, middle, and internal ear. The external ear and middle ear are mainly concerned with the transfer of sound to the internal ear, which contains the organ for equilibrium as well as for hearing. The tympanic membrane separates the external ear from the middle ear. The pharyngotympanic tube joins the middle ear to the nasopharynx.

EXTERNAL EAR

The external ear is composed of the shell-like auricle (pinna), which collects sound, and the external acoustic meatus (ear canal), which conducts sound to the tympanic membrane.

AURICLE

The auricle (L. auris, ear) is composed of an irregularly shaped plate of elastic cartilage that is covered by thin skin. The auricle has several depressions and elevations. The concha of the auricle is the deepest depression. The elevated margin of the auricle is the helix. The non-cartilaginous lobule (lobe) consists of fibrous tissue, fat, and blood vessels. It is easily pierced for taking small blood samples and inserting earrings. The tragus (G. tragos, goat; alluding to the hairs that tend to grow from this formation, like a goat’s beard) is a tongue-like projection overlapping the opening of the external acoustic meatus.

The arterial supply to the auricle is derived mainly from the posterior auricular and superficial temporal arteries.

The main nerves to the skin of the auricle are the great auricular and auriculotemporal nerves. The great auricular nerve supplies the cranial (medial) surface (commonly called the “back of the ear”) and the posterior part (helix, antihelix, and lobule) of the lateral surface (“front”). The auriculotemporal nerve, a branch of CN V3, supplies the skin of the auricle anterior to the external acoustic meatus. Minor contributions of embryological significance are made to the skin of the concha and its eminence by the vagus and facial nerves.

The lymphatic drainage of the auricle is as follows: the lateral surface of the superior half of the auricle drains to the superficial parotid lymph nodes; the cranial surface of the superior half of the auricle drains to the mastoid lymph nodes and deep cervical lymph nodes; and the remainder of the auricle, including the lobule, drains into the superficial cervical lymph nodes.

EXTERNAL ACOUSTIC MEATUS

The external acoustic meatus is an ear canal that leads inward through the tympanic part of the temporal bone from the auricle to the tympanic membrane, a distance of 2–3 cm in adults. The lateral third of this slightly S-shaped canal is cartilaginous and is lined with skin that is continuous with the auricular skin. The medial two thirds of the meatus is bony and lined with thin skin that is continuous with the external layer of the tympanic membrane. The ceruminous and sebaceous glands in the subcutaneous tissue of the cartilaginous part of the meatus produce cerumen (earwax).

The tympanic membrane, approximately 1 cm in diameter, is a thin, oval semitransparent membrane at the medial end of the external acoustic meatus. This membrane forms a partition between the external acoustic meatus and the tympanic cavity of the middle ear. The tympanic membrane is covered with thin skin externally and mucous membrane of the middle ear internally. Viewed through an otoscope, the tympanic membrane has a concavity toward the external acoustic meatus with a shallow, cone-like central depression, the peak of which is the umbo. The central axis of the tympanic membrane passes perpendicularly through the umbo like the handle of an umbrella, running anteriorly and inferiorly as it runs laterally. Thus, the tympanic membrane is oriented like a mini radar or satellite dish positioned to receive signals coming from the ground in front and to the side of the head.

Superior to the lateral process of the malleus (one of the small ear bones, or auditory ossicles, of the middle ear), the membrane is thin and is called the pars flaccida (flaccid part. It lacks the radial and circular fibers present in the remainder of the membrane, called the pars tensa (tense part). The flaccid part forms the lateral wall of the superior recess of the tympanic cavity.

The tympanic membrane moves in response to air vibrations that pass to it through the external acoustic meatus. Movements of the membrane are transmitted by the auditory ossicles through the middle ear to the internal ear. The external surface of the tympanic membrane is supplied mainly by the auriculotemporal nerve, a branch of CN V3. Some innervation is supplied by a small auricular branch of the vagus (CN X). The internal surface of the tympanic membrane is supplied by the glossopharyngeal nerve (CN IX).

MIDDLE EAR

The tympanic cavity or cavity of the middle ear is the narrow air-filled chamber in the petrous part of the temporal bone. The cavity has two parts: the tympanic cavity proper, the space directly internal to the tympanic membrane, and the epitympanic recess, the space superior to the membrane. The tympanic cavity is connected anteromedially with the nasopharynx by the pharyngotympanic tube and posterosuperiorly with the mastoid cells through the mastoid antrum. The tympanic cavity is lined with mucous membrane that is continuous with the lining of the pharyngotympanic tube, mastoid cells, and mastoid antrum.

The contents of the middle ear are the:

• Auditory ossicles (malleus, incus, and stapes).
• Stapedius and tensor tympani muscles.
• Chorda tympani nerve, a branch of CN VII
• Tympanic plexus of nerves.

WALLS OF TYMPANIC CAVITY

The middle ear is shaped like a lozenge or narrow box with concave sides. It has six walls.

1. The tegmental wall (roof) is formed by a thin plate of bone, the tegmen tympani, which separates the tympanic cavity from the dura mater on the floor of the middle cranial fossa.
2. The jugular wall (floor) is formed by a layer of bone that separates the tympanic cavity from the superior bulb of the internal jugular vein.
3. The membranous (lateral) wall is formed almost entirely by the peaked convexity of the tympanic membrane; superiorly it is formed by the lateral bony wall of the epitympanic recess. The handle of the malleus is attached to the tympanic membrane, and its head extends into the epitympanic recess.
4. The labyrinthine (medial) wall (medial wall) separates the tympanic cavity from the internal ear. It also features the promontory of the labyrinthine wall, formed by the initial part (basal turn) of the cochlea, and the oval and round windows, which, in a dry cranium, communicate with the internal ear.
5. The mastoid wall (posterior wall) features an opening in its superior part, the aditus (L. access) to the mastoid antrum, connecting the tympanic cavity to the mastoid cells; the canal for the facial nerve descends between the posterior wall and the antrum, medial to the aditus.
6. The anterior carotid wall separates the tympanic cavity from the carotid canal; superiorly, it has the opening of the pharyngotympanic tube and the canal for the tensor tympani.

The mastoid antrum is a cavity in the mastoid process of the temporal bone. The antrum (L. from G., cave), like the tympanic cavity, is separated from the middle cranial fossa by a thin plate of the temporal bone, called the tegmen tympani. This structure forms the tegmental wall (roof) for the ear cavities and is also part of the floor of the lateral part of the middle cranial fossa. The mastoid antrum is the common cavity into which the mastoid cells open. The antrum and mastoid cells are lined by mucous membrane that is continuous with the lining of the middle ear. Antero-inferiorly, the antrum is related to the canal for the facial nerve.

PHARYNGOTYMPANIC TUBE

The pharyngotympanic tube (auditory tube) connects the tympanic cavity to the nasopharynx, where it opens posterior to the inferior nasal meatus. The posterolateral third of the tube is bony, and the remainder is cartilaginous. The pharyngotympanic tube is lined by mucous membrane that is continuous posteriorly with that of the tympanic cavity and anteriorly with that of the nasopharynx.

The function of the pharyngotympanic tube is to equalize pressure in the middle ear with the atmospheric pressure, thereby allowing free movement of the tympanic membrane. By allowing air to enter and leave the tympanic cavity, this tube balances the pressure on both sides of the membrane. Because the walls of the cartilaginous part of the tube are normally in apposition, the tube must be actively opened. The tube is opened by the expanding girth of the belly of the levator veli palatini as it contracts longitudinally, pushing against one wall while the tensor veli palatini pulls on the other. Because these are muscles of the soft palate, equalizing pressure (“popping the eardrums”) is commonly associated with activities such as yawning and swallowing.

The arteries of the pharyngotympanic tube are derived from the ascending pharyngeal artery, a branch of the external carotid artery, and the middle meningeal artery and artery of the pterygoid canal, ranches of the maxillary artery.

The veins of the pharyngotympanic tube drain into the pterygoid venous plexus. Lymphatic drainage of the tube is to the deep cervical lymph nodes.

The nerves of the pharyngotympanic tube arise from the tympanic plexus, which is formed by fibers of the glossopharyngeal nerve (CN IX). Anteriorly, the tube also receives fibers from the pterygopalatine ganglion.

AUDITORY OSSICLES

The auditory ossicles form a mobile chain of small bones across the tympanic cavity from the tympanic membrane to the oval window (L. fenestra vestibuli), an oval opening on the labyrinthine wall of the tympanic cavity leading to the vestibule of the bony labyrinth. These ossicles are the first bones to be fully ossified during development and are essentially mature at birth. The bone from which they are formed is exceptionally dense (hard). The ossicles are covered with the mucous membrane lining the tympanic cavity; but unlike other bones, they lack a surrounding layer of osteogenic periosteum.

MALLEUS.

The malleus (L. a hammer) attaches to the tympanic membrane. The rounded superior head of the malleus lies in the epitympanic recess. The neck of the malleus lies against the flaccid part of the tympanic membrane, and the handle of the malleus is embedded in the tympanic membrane, with its tip at the umbo; thus, the malleus moves with the membrane. The head of the malleus articulates with the incus; the tendon of the tensor tympani inserts into its handle near the neck. The chorda tympani crosses the medial surface of the neck of the malleus. The malleus functions as a lever, with the longer of its two processes and its handle attached to the tympanic membrane.

Incus. The incus (L. an anvil) is located between the malleus and the stapes and articulates with them. It has a body and two limbs. Its large body lies in the epitympanic recess, where it articulates with the head of the malleus. The long limb lies parallel to the handle of the malleus, and its interior end articulates with the stapes by way of the lenticular process, a medially directed projection. The short limb is connected by a ligament to the posterior wall of the tympanic cavity.

STAPES.

The stapes (L. stirrup) is the smallest ossicle. It has a head, two limbs, and a base. Its head, directed laterally, articulates with the incus. The base (footplate) of the stapes fits into the oval window on the medial wall of the tympanic cavity. The oval base is attached to the margins of the oval window. The base of the stapes is considerably smaller than the tympanic membrane; as a result, the vibratory force of the stapes is increased approximately 10 times over that of the tympanic membrane. Consequently, the auditory ossicles increase the force but decrease the amplitude of the vibrations transmitted from the tympanic membrane through the ossicles to the internal ear.

MUSCLES ASSOCIATED WITH AUDITORY OSSICLES.

Two muscles dampen or resist movements of the auditory ossicles; one also dampens movements (vibration) of the tympanic membrane. The tensor tympani is a short muscle that arises from the superior surface of the cartilaginous part of the pharyngotympanic tube, the greater wing of the sphenoid, and the petrous part of the temporal bone. The muscle inserts into the handle of the malleus. The tensor tympani pulls the handle medially, tensing the tympanic membrane and reducing the amplitude of its oscillations. This action tends to prevent damage to the internal ear when one is exposed to loud sounds. The tensor tympani is supplied by the mandibular nerve (CN V3).

The stapedius is a tiny muscle inside the pyramidal eminence (pyramid), a hollow, cone-shaped prominence on the posterior wall of the tympanic cavity. Its tendon enters the tympanic cavity by emerging from a pinpoint foramen in the apex of the eminence and inserts on the neck of the stapes. The stapedius pulls the stapes posteriorly and tilts its base in the oval window, thereby tightening the anular ligament and reducing the oscillatory range. It also prevents excessive movement of the stapes. The nerve to the stapedius arises from the facial nerve (CN VII).

INTERNAL EAR

The internal ear contains the vestibulocochlear organ concerned with the reception of sound and the maintenance of balance. Buried in the petrous part of the temporal bone, the internal ear consists of the sacs and ducts of the membranous labyrinth. The membranous labyrinth, containing endolymph, is suspended within the perilymph-filled bony labyrinth, either by delicate filaments similar to the filaments of arachnoid mater that traverse the subarachnoid space or by the substantial spiral ligament. It does not float. These fluids are involved in stimulating the end organs for balance and hearing, respectively.

BONY LABYRINTH

The bony labyrinth is a series of cavities (cochlea, vestibule, and semicircular canals) contained within the otic capsule of the petrous part of the temporal bone. The otic capsule is made of bone that is denser than the remainder of the petrous temporal bone and can be isolated (carved) from it using a dental drill. The otic capsule is often erroneously illustrated and identified as being the bony labyrinth. However, the bony labyrinth is the fluid-filled space, which is surrounded by the otic capsule, and is most accurately represented by a cast of the otic capsule after removal of the surrounding bone.

Cochlea. The cochlea is the shell-shaped part of the bony labyrinth that contains the cochlear duct, the part of the internal ear concerned with hearing. The spiral canal of the cochlea begins at the vestibule and makes 2.5 turns around a bony core, the modiolus, the cone-shaped core of spongy bone about which the spiral canal of the cochlea turns. The modiolus contains canals for blood vessels and for distribution of the branches of the cochlear nerve. The apex of the cone-shaped modiolus, like the axis of the tympanic membrane, is directed laterally, anteriorly, and inferiorly. The large basal turn of the cochlea produces the promontory of the labyrinthine wall of the tympanic cavity. At the basal turn, the bony labyrinth communicates with the subarachnoid space superior to the jugular foramen through the cochlear aqueduct. It also features the round window (L. fenestra cochleae), closed by the secondary tympanic membrane.

VESTIBULE OF BONY LABYRINTH.

The vestibule of the bony labyrinth is a small oval chamber (approximately 5 mm long) that contains the utricle and saccule and parts of the balancing apparatus (vestibular labyrinth). The vestibule features the oval window on its lateral wall, occupied by the base of the stapes. The vestibule is continuous with the bony cochlea anteriorly, the semicircular canals posteriorly, and the posterior cranial fossa by the vestibular aqueduct.

The aqueduct extends to the posterior surface of the petrous part of the temporal bone, where it opens posterolateral to the internal acoustic meatus. The vestibular aqueduct transmits the endolymphatic duct and two small blood vessels.

SEMICIRCULAR CANALS.

The semicircular canals (anterior, posterior, and lateral) communicate with the vestibule of the bony labyrinth. The canals lie posterosuperior to the vestibule into which they open; they are set at right angles to each other. The canals occupy three planes in space. Each semicircular canal forms approximately two thirds of a circle, and is approximately 1.5 mm in diameter, except at one end where there is a swelling, the bony ampulla. The canals have only five openings into the vestibule because the anterior and posterior canals have one limb common to both. Lodged within the canals are the semicircular ducts.

MEMBRANOUS LABYRINTH

The membranous labyrinth consists of a series of communicating sacs and ducts that are suspended in the bony labyrinth. The labyrinth contains endolymph, a watery fluid similar in composition to intracellular fluid, thus differing in composition from the surrounding perilymph (which is like extracellular fluid) that fills the remainder of the bony labyrinth. The membranous labyrinth—composed of two divisions, the vestibular labyrinth and the cochlear labyrinth—consists of more parts than does the bony labyrinth:

• Vestibular labyrinth: utricle and saccule, two small communicating sacs in the vestibule of the bony labyrinth.
• Three semicircular ducts in the semicircular canals.
• Cochlear labyrinth: cochlear duct in the cochlea.

The spiral ligament, a spiral thickening of the periosteal lining of the cochlear canal, secures the cochlear duct to the spiral canal of the cochlea. The remainder of the membranous labyrinth is suspended by delicate filaments that traverse the perilymph.

The semicircular ducts open into the utricle through five openings, reflective of the way the surrounding semicircular canals open into the vestibule. The utricle communicates with the saccule through the utriculosaccular duct, from which the endolymphatic duct arises. The saccule is continuous with the cochlear duct through the ductus reuniens, a uniting duct. The utricle and saccule have specialized areas of sensory epithelium called maculae. The macula of the utricle (L. macula utriculi) is in the floor of the utricle, parallel with the base of the cranium, whereas the macula of the saccule (L. macula sacculi) is vertically placed on the medial wall of the saccule.

The hair cells in the maculae are innervated by fibers of the vestibular division of the vestibulocochlear nerve. The primary sensory neurons are in the vestibular ganglia, which are in the internal acoustic meatus.

The endolymphatic duct traverses the vestibular aqueduct and emerges through the bone of the posterior cranial fossa, where it expands into a blind pouch called the endolymphatic sac. The endolymphatic sac is located under the dura mater on the posterior surface of the petrous part of the temporal bone. The sac is a storage reservoir for excess endolymph, formed by the blood capillaries in the membranous labyrinth.

SEMICIRCULAR DUCTS.

Each semicircular duct has an ampulla at one end containing a sensory area, the ampullary crest (L. crista ampullari). The crests are sensors for recording movements of the endolymph in the ampulla resulting from rotation of the head in the plane of the duct. The hair cells of the crests, like those of the maculae, stimulate primary sensory neurons, whose cell bodies are in the vestibular ganglia.

COCHLEAR DUCT.

The cochlear duct is a spiral tube, closed at one end and triangular in cross section. The duct is firmly suspended across the cochlear canal between the spiral ligament on the external wall of the cochlear canal and the osseous spiral lamina of the modiolus. Spanning the spiral canal in this manner, the endolymph-filled cochlear duct divides the perilymph-filled spiral canal into two channels that are continuous at the apex of the cochlea at the helicotrema, a semilunar communication at the apex of the cochlea.

Waves of hydraulic pressure created in the perilymph of the vestibule by the vibrations of the base of the stapes ascend to the apex of the cochlea by one channel, the scala vestibuli. The pressure waves then pass through the helicotrema and descend back to the basal turn of the cochlea by the other channel, the scala tympani. Here, the pressure waves again become vibrations, this time of the secondary tympanic membrane in the round window, and the energy initially received by the (primary) tympanic membrane is finally dissipated into the air of the tympanic cavity.

The roof of the cochlear duct is formed by the vestibular membrane. The floor of the duct is also formed by part of the duct, the basilar membrane, plus the outer edge of the osseous spiral lamina. The receptor of auditory stimuli is the spiral organ (of Corti), situated on the basilar membrane. It is overlaid by the gelatinous tectorial membrane.

The spiral organ contains hair cells, the tips of which are embedded in the tectorial membrane. The organ is stimulated to respond by deformation of the cochlear duct induced by the hydraulic pressure waves in the perilymph, which ascend and descend in the surrounding scalae vestibuli and tympani.

INTERNAL ACOUSTIC MEATUS

The internal acoustic meatus is a narrow canal that runs laterally for approximately 1 cm within the petrous part of the temporal bone. The internal acoustic meatus opening is in the posteromedial part of this bone, in line with the external acoustic meatus. The internal acoustic meatus is closed laterally by a thin, perforated plate of bone that separates it from the internal ear. Through this plate pass the facial nerve (CN VI), the vestibulocochlear nerve (CN VIII) and its divisions, and blood vessels. The vestibulocochlear nerve divides near the lateral end of the internal acoustic meatus into two parts: a cochlear nerve and a vestibular nerve.

Clinically oriented anatomy 7^th ed.