USMLE Step 1 Qs (3)

The venous drainage from each adrenal gland tends to be through a single vein. The left adrenal gland usually drains into the left renal vein superior to the point where the gonadal vein enters the left renal vein. The left adrenal vein usually anastomoses with the hemiazygos vein and may provide an important route of collateral venous return. The right adrenal gland usually drains directly into the inferior vena cava.

Obstruction of any portion of the biliary tree will produce symptoms of gallbladder attack. If the common hepatic duct or bile duct is occluded by stone or tumor, biliary stasis with accompanying jaundice occurs. In addition, blockage of the duodenal papilla (of Vater), distal to the juncture of the bile duct with the pancreatic duct, can lead to complicating pancreatitis. If only the cystic duct is obstructed, jaundice will not occur because bile may flow freely from the liver to the duodenum. Bile duct obstruction also may arise as a result of pressure exerted on the duct by an external mass, such as a tumor in the head of the pancreas.

The serratus anterior muscle (protractor and stabilizer of the scapula) is innervated by the long thoracic nerve (of Bell), which arises from roots C5 to C7 of the brachial plexus. During modified radical mastectomy, this nerve is usually spared to maintain shoulder function. However, its location places it in jeopardy during the lymphatic resection. The suprascapular nerves are sensory branches of the cervical plexus. The axillary nerve, deep in the brachial portion of the axilla, innervates the deltoid muscle. The thoracodorsal nerve, which arises from the posterior cord of the brachial plexus, innervates the latissimus dorsi. The lower subscapular nerve innervates the teres major muscle and a portion of the subscapularis muscle.

The umbilical region is innervated by the tenth intercostal nerve. The afferent nerve fibers from the jejunum and ileum as well as from the ascending colon and transverse colon travel through the superior mesenteric plexus and along the lesser splanchnic nerve to spinal nerves T10 and T11. Thus, pain originating from these portions of the gastrointestinal tract will refer pain to the umbilical region. The ascending colon and descending colon, which are secondarily retroperitoneal, are unlikely to be involved in the umbilical herniation. The mobile transverse colon could be involved, but the referred pain would tend to be subumbilical, not periumbilical.

The photomicrograph shows the histology of the adrenal gland showing the cortex (*) and medulla (– – –), which both release stress-related hormones [i.e., glucocorticoids and catecholamines (norepinephrine and epinephrine)]. The adrenal cortex originates from the intermediate mesoderm, whereas the adrenal medulla forms from neural crest. Adrenocortical cells are under the influence of corticotrophs in the anterior pituitary. Adrenocortical cells import cholesterol and acetate and produce the hormones shown in the table below. The zona glomerulosa (A) is found immediately beneath the capsule (E) and is followed by the zona fasciculata (B) and zona reticularis (C) as one moves toward the medulla (D). However, in all zones the cells do not store appreciable quantities of hormones, there is an absence of secretory granules, and the steroid hormones are released by diffusion through the plasma membrane without use of the exocytotic process used by most glands, including the adrenal medulla. The cells of the adrenal medulla (D) may be considered as modified postganglionic sympathetic neurons. Adrenal medullary cells synthesize and secrete norepinephrine, epinephrine, and enkephalins in response to stimulation of preganglionic sympathetic fibers that travel through the abdomen in the splanchnic nerves and innervate the gland. The adrenal cortical hormones are viewed as essential for life because of their regulation of metabolism. In Addison's disease there is a progressive destruction (hypotrophy) of the adrenal cortex (zones A, B, and C). The result in the patient is asthenia (lack of strength, overall weakness, and fatigue), anorexia, nausea, vomiting, weight loss, hypotension, and low blood sugar.

The adrenal hormones are listed in the table below:
Zone Secretion Target Regulartory Factors
Zona glomerulosa Mineralocorticoids (aldosterone) Collecting tubules Angiotensin II
Zona fasciculata Glucocorticoids (cortisol, hydro-cortisone) and weak androgens Gluconeogenesis by the liver ACTH (adrenocortico-tropic hormone)
Zona reticularis Glucocorticoids and weak androgens Androgens are precursors of estradiol in the fetus ACTH
Medulla Norepinephrine and epinephrine Preparation for "flight or fight" Preganglionic sympathetic fibers from the splanchnic nerves

The short gastric veins are branches of the splenic vein and, therefore, would experience the portal pressure. The short gastric veins also anastomose with the esophageal veins and produce esophageal varices. The hepatic vein, between the liver and inferior vena cava, drains the liver and is not part of the portal system. There are no communications between the portal system and the renal or suprarenal veins.

During the cardiac cycle, differential pressures between the atria and ventricles open and close the atrioventricular valves. The papillary muscles attach the chordae tendineae to the heart wall and provide an important dynamic mechanism to ensure the competence of the valves. During the ejection phase of ventricular systole, shortening of the papillary muscle compensates for the decrease in the ventricular chamber size and thereby, prevents eversion of the atrioventricular valve leaflets, thereby preventing regurgitation during ventricular systole.

The adrenal medulla is innervated from thoracic levels of the spinal cord mediated by preganglionic sympathetic nerve fibers traveling in the lesser and least splanchnic nerves, with some contribution from the greater splanchnic and lumbar splanchnic nerves. Because both the adrenal medulla and postganglionic sympathetic neurons are adrenergic and derived from neural crest tissue, the homology of the chromaffin cells and postganglionic sympathetic neurons is apparent. There appears to be no parasympathetic innervation to the adrenal medulla and no innervation whatever to the adrenal cortex.

The lymphatic drainage of the mammary gland, which follows the path of its blood supply, generally parallels the tributaries of the axillary, internal thoracic (mammary), thoracoacromial, and intercostal vessels. Because about 75% of the breast lies lateral to the nipple, the more significant lateral and inferior portions of the breast drain toward the axillary nodes. The smaller medial portion drains to the parasternal lymphatic chain paralleling the internal thoracic vessels, whereas the very small superior portion drains toward the nodes associated with the thoracoacromial trunk and the supraclavicular nodes.

The artery labeled D in the diagram accompanying the question represents the posterior interventricular (descending) artery, which supplies blood to the posterior portions of the interventricular septum as well as to the posterior wall of the right ventricle. This artery usually is a branch of the right coronary artery, and the diagnosis of this patient's disorder is consistent with the results of the ECG, which indicates a posterior septal infarct. The anterior interventricular artery (C) arises from the left coronary artery (A) and supplies the anterior portion of the interventricular septum and the anterior walls of both ventricles. The posterior interventricular artery (D) usually anastomoses with the anterior interventricular artery (C) near the apex of the heart. The circumflex artery (B) circles toward the back of the heart in the coronary sulcus and may occasionally give rise to the posterior interventricular artery (D). The right marginal artery (E) is a branch of the right coronary artery.

The gallbladder lies on the inferior surface of the liver between the right and quadrate lobes. The caudate lobe lies posteriorly between the right and left lobes. The falciform ligament, a portion of the lesser omentum, attaches to the liver at the incisura between the quadrate and left lobes as well as along the fissure for the round ligament. Toward the superior surface of the liver, the falciform ligament splits to form the left and right coronary ligaments, which define the bare area of the liver. The coronary ligaments come together again to form the gastrohepatic ligament of the lesser omentum.

The great cardiac vein accompanies the anterior interventricular (descending) artery. The anterior cardiac veins pass across the right coronary sulcus to drain directly into the right atrium. The middle cardiac vein lies in the posterior interventricular sulcus with the posterior descending artery. The small cardiac vein accompanies the right marginal vein and the right coronary artery. The coronary sinus, accompanying the circumflex artery in the left coronary sulcus, receives the great, middle, and small cardiac veins before draining into the right atrium.

Several abdominal structures are involved in the formation of the spermatic cord. The deep fascia contributes the external spermatic fascia. Although some references include the external oblique muscle or aponeurosis, no contribution is derived from that layer owing to a hiatus in the aponeurosis. The cremaster muscle, a contribution of the internal oblique muscle, joins the spermatic cord as the inguinal canal passes through that layer. The transversus abdominis muscle, which usually terminates as the falx inguinalis just superior to the deep ring, contributes to the cremaster muscle in less than 5% of all males. The transversalis fascia contributes the internal spermatic fascia.

Perforating branches of the deep femoral artery are the principal blood supply to the posterior thigh. The other arteries supply anterior, medial, and gluteal regions of the thigh.

The cell marked with a star is a Leydig cell (i.e., interstitial cell) and is regulated by luteinizing hormone (LH), also known as interstitial cell-stimulating hormone (ICSH), secreted by gonadotrophs in the anterior pituitary. Leydig cells are located between seminiferous tubules and are responsible for the production of testosterone. Sertoli cells function in a nutritive and supportive role somewhat analogous to the glial cells of the CNS. The Sertoli cells produce inhibin, which feeds back on the anterior pituitary and hypothalamus to regulate FSH release. Testosterone is modified by binding to androgen-binding protein (ABP), which is synthesized by the Sertoli cells. The testosterone is necessary for the maintenance of spermatogenesis as well as the male ducts and accessory glands. ABP is regulated by FSH, testosterone, and inhibin. Sertoli cells have extensive tight (occluding) junctions between them that form the blood-testis barrier. Sertoli cells communicate with adjacent cells through gap junctions and extend from outside the blood-testis barrier (basal portion) to luminal to the blood-testis barrier (apical portion). During spermatogenesis, derivatives of spermatocytes cross from the basal to the adluminal compartment across the zonula occludens between adjacent Sertoli cells. Each Sertoli cell is, therefore, associated with multiple spermatogenic cells.

The testis is composed of seminiferous tubules containing a number of spermatogenic cells undergoing spermatogenesis and spermiogenesis. The cells labeled with the arrowheads are spermatogonia, the derivatives of the embryonic primordial germ cells. The spermatogonia are of three types (not distinguishable in the photomicrograph): type A dark cells (Ad), type A pale cells (Ap), and type B cells. Type Ad cells are precursors that divide to form Ad and Ad progeny or Ad and Ap progeny. The Ap cells give rise to B spermatogonia that are capable of differentiating into primary spermatocytes. These cells comprise the basal layer and undergo mitosis (spermatocytogenesis) to form primary spermatocytes, which have distinctive clumped or coarse chromatin (marked by arrows). Secondary spermatocytes are formed during the first meiotic division and exist for only a short period of time because there is no lag period before entry into the second meiotic division that results in the formation of spermatids. The spermatids begin as round structures and elongate with the formation of the flagellum. This last part of seminiferous tubule function is the differentiation of sperm from spermatocytes (spermiogenesis) and is complete with the release of mature sperm into the lumen of the tubule.

The fasciculus gracilis consists of afferent fibers carrying information concerning two-point discrimination, vibration, and joint/limb position from the lower extremities and inferior trunk. The cuneate fasciculus (fasciculus cuneatus) carries the same modalities from the superior trunk and upper extremities. Classically, light touch is the modality assigned to afferents in the anterior spinothalamic tract, whereas afferents of the lateral spinothalamic tract convey information regarding pain and temperature. Fibers in the cuneocerebellar tract convey proprioception from the upper extremity.

The rectus abdominis muscle receives an abundant collateral blood supply. The nerve supply to the rectus abdominis muscle is derived from abdominal extensions of the lower seven intercostal nerves and from the iliohypogastric nerve. These nerves run between the internal oblique and transversus abdominis muscles to reach the lateral border of the rectus sheath, which they pierce to reach the rectus abdominis muscle. Consequently, a subcostal incision from the xiphisternal angle to the anterior axillary line is apt to sever one or two of these nerves and thus paralyze a significant portion of the ipsilateral rectus abdominis muscle. An affected patient may be predisposed to subsequent abdominal herniation. Although the direction of an incision along the costal margin is perforce perpendicular to the dermal cleavage lines (of Langer) and thus may produce discomfort and healing with significant scarring, such an incision is justified by the required operative exposure that it provides.

The jejunum and ileum receive their principal blood supply from the superior mesenteric artery. A strong collateral circulation is derived from the superior pancreatic artery, a branch of the pancreaticoduodenal artery that arises from the hepatic branch of the celiac artery. The superior pancreatic artery anastomoses with the inferior pancreatic artery, the first branch of the superior mesenteric artery. The collateral circulation is weak between the right colic artery and the ileal branches. There are no possibilities for superior mesenteric anastomoses from the splenic, the descending branch of the left colic, or renal arteries.

The diffuse central abdominal pain in the patient presented is probably referred pain from the loop of small bowel incarcerated within the herniated peritoneal sac. Compression of the bowel results in compromise of the blood supply and subsequent ischemic necrosis. The visceral afferent fibers from the distal small bowel travel along the blood vessels to reach the superior mesenteric plexus and lesser splanchnic nerves, which they follow to the T10–T11 levels of the spinal cord. The pain, therefore, is referred to (appears as if originating from) the T10–T11 dermatomes, which supply the umbilical region. Because the gut develops as a midline structure, visceral pain tends to be centrally located regardless of the adult location of any particular region of the gut. As a result of dilation of the inguinal canal by the hernial sac, however, the patient also experiences localized somatic pain mediated by the iliohypogastric, ilioinguinal, and genitofemoral nerves.

Because large and important neurovascular structures pass between the clavicle and first rib, including the subclavian artery, clavicular fracture may produce life-threatening bleeding into the pleural cavity. The axillary artery is the continuation of the subclavian after it has cleared the first rib, so neither this vessel nor its thoracoacromial branch is likely to be threatened by clavicular fracture. There is no brachiocephalic artery on the left side, and on the right its terminal point is marked by its bifurcation into common carotid and subclavian arteries proximal to the fracture site.

In a coronary bypass procedure, the distal end of the vein graft is anastomosed to the aorta so that the presence of a valve or valve leaflets in the graft will not obstruct the flow of coronary blood. In recent years, the reversed saphenous vein graft from the calf has been the choice for this procedure. This vein is closer in size to the coronary arteries than one taken from the thigh

The ureter, lying just medial to the internal iliac artery in the deep pelvis, passes from posterior to anterior immediately inferior to the lateral cervical ligament. This ligament contains the uterine artery and vein to which the ureters pass inferior approximately midway along their course between internal iliac artery and uterus. The ureter continues inferior to the anterior portion of the lateral cervical ligament (where it can sometimes be palpated through the walls of the vagina at the lateral fornices) to gain access to the base of the urinary bladder. The close association between uterine vessels and ureter is of major importance during surgical procedures in the female pelvis.

The deep incisure in the inferior border of the pedicle ensures that the spinal nerve associated with that vertebra will exit through the intervertebral foramen well above the intervertebral disk so that it will not be affected by a herniation at that level. However, a posterolateral herniation (the usual direction) will impinge on the next lower nerve as it courses toward its associated intervertebral foramen. In this case, pain was distributed along the medial side of the leg and foot as far as the great toe—the distribution of the saphenous branch of the femoral nerve (L4). Herniation of the third lumbar intervertebral disk between vertebral bodies L3–L4 would affect nerve L4.

Innervation of the bronchial smooth muscle is mediated by parasympathetic neurons carried by the vagus nerve. These nerves also stimulate secretion from the bronchial glands. Excessive vagal activity may initiate bronchospasm or the asthmatic syndrome.

The photomicrograph shows the histology of the adrenal gland showing the cortex (*) and medulla (– – –), which both release stress-related hormones [i.e., glucocorticoids and catecholamines (norepinephrine and epinephrine)]. The adrenal cortex originates from the intermediate mesoderm, whereas the adrenal medulla forms from neural crest. Adrenocortical cells are under the influence of corticotrophs in the anterior pituitary. Adrenocortical cells import cholesterol and acetate and produce the hormones shown in the table below. The zona glomerulosa (A) is found immediately beneath the capsule (E) and is followed by the zona fasciculata (B) and zona reticularis (C) as one moves toward the medulla (D). However, in all zones the cells do not store appreciable quantities of hormones, there is an absence of secretory granules, and the steroid hormones are released by diffusion through the plasma membrane without use of the exocytotic process used by most glands, including the adrenal medulla. The cells of the adrenal medulla (D) may be considered as modified postganglionic sympathetic neurons. Adrenal medullary cells synthesize and secrete norepinephrine, epinephrine, and enkephalins in response to stimulation of preganglionic sympathetic fibers that travel through the abdomen in the splanchnic nerves and innervate the gland. The adrenal cortical hormones are viewed as essential for life because of their regulation of metabolism. In Addison's disease there is a progressive destruction (hypotrophy) of the adrenal cortex (zones A, B, and C). The result in the patient is asthenia (lack of strength, overall weakness, and fatigue), anorexia, nausea, vomiting, weight loss, hypotension, and low blood sugar.
The adrenal hormones are listed in the table below:
Zone Secretion Target Regulartory Factors
Zona glomerulosa Mineralocorticoids (aldosterone) Collecting tubules Angiotensin II
Zona fasciculata Glucocorticoids (cortisol, hydro-cortisone) and weak androgens Gluconeogenesis by the liver ACTH (adrenocortico-tropic hormone)
Zona reticularis Glucocorticoids and weak androgens Androgens are precursors of estradiol in the fetus ACTH
Medulla Norepinephrine and epinephrine Preparation for "flight or fight" Preganglionic sympathetic fibers from the splanchnic nerves

The cell marked with a star is a Leydig cell (i.e., interstitial cell) and is regulated by luteinizing hormone (LH), also known as interstitial cell-stimulating hormone (ICSH), secreted by gonadotrophs in the anterior pituitary. Leydig cells are located between seminiferous tubules and are responsible for the production of testosterone. Sertoli cells function in a nutritive and supportive role somewhat analogous to the glial cells of the CNS. The Sertoli cells produce inhibin, which feeds back on the anterior pituitary and hypothalamus to regulate FSH release. Testosterone is modified by binding to androgen-binding protein (ABP), which is synthesized by the Sertoli cells. The testosterone is necessary for the maintenance of spermatogenesis as well as the male ducts and accessory glands. ABP is regulated by FSH, testosterone, and inhibin. Sertoli cells have extensive tight (occluding) junctions between them that form the blood-testis barrier. Sertoli cells communicate with adjacent cells through gap junctions and extend from outside the blood-testis barrier (basal portion) to luminal to the blood-testis barrier (apical portion). During spermatogenesis, derivatives of spermatocytes cross from the basal to the adluminal compartment across the zonula occludens between adjacent Sertoli cells. Each Sertoli cell is, therefore, associated with multiple spermatogenic cells.

The testis is composed of seminiferous tubules containing a number of spermatogenic cells undergoing spermatogenesis and spermiogenesis. The cells labeled with the arrowheads are spermatogonia, the derivatives of the embryonic primordial germ cells. The spermatogonia are of three types (not distinguishable in the photomicrograph): type A dark cells (Ad), type A pale cells (Ap), and type B cells. Type Ad cells are precursors that divide to form Ad and Ad progeny or Ad and Ap progeny. The Ap cells give rise to B spermatogonia that are capable of differentiating into primary spermatocytes. These cells comprise the basal layer and undergo mitosis (spermatocytogenesis) to form primary spermatocytes, which have distinctive clumped or coarse chromatin (marked by arrows). Secondary spermatocytes are formed during the first meiotic division and exist for only a short period of time because there is no lag period before entry into the second meiotic division that results in the formation of spermatids. The spermatids begin as round structures and elongate with the formation of the flagellum. This last part of seminiferous tubule function is the differentiation of sperm from spermatocytes (spermiogenesis) and is complete with the release of mature sperm into the lumen of the tubule.

The palpebral portion of the orbicularis oculi muscle (innervated by the zygomatic branch of the facial nerve) produces the blink, whereas the orbital portion is involved in "scrunching" the eye shut. The buccal branch of the facial nerve innervates muscles of facial expression (including the buccinator muscle) between the eye and the mouth, whereas the buccal branch of the trigeminal nerve is sensory. The levator palpebrae superioris muscle, which elevates the upper eyelid, is innervated by the oculomotor nerve, whereas the involuntary superior tarsal muscle is supplied by sympathetic nerves.

All of the muscles listed above are innervated by the posterior interosseus branch of the radial nerve (the terminal part of the deep radial nerve). Extensor carpi radialis longus, however, is innervated by a muscular branch of the radial nerve proximal to the origin of the deep branch. Its function would, therefore, be preserved in entrapment of the posterior interosseus nerve.

The peroneus (fibularis) brevis, a pronator and everter of the foot, inserts into the tubercle at the base of the fifth metatarsal. The peroneus longus passes under the tarsal arch to insert onto the plantar aspect of the first metatarsal. The tibialis posterior inserts onto the navicular bone, whereas the tibialis anterior inserts into the first cuneiform and first metatarsal. The abductor digiti minimi inserts onto the proximal phalanx of the fifth toe.

The afferent limb of the cardiac reflex is mediated by the carotid branch of the glossopharyngeal nerve (CN IX) from the aortic body and sinus as well as by the vagus nerve (CN X) from the aortic body. The efferent limb, which is carried by the sympathetic division of the autonomic nervous system, mediates increases in heart rate and strength of heart beat through release of norepinephrine at the postganglionic effector site. The sympathetic cardiac accelerator fibers, affecting primarily the ventricles, are derived from the superior, middle, and inferior cervical ganglia (cervical cardiac nerves) as well as from the upper four thoracic ganglia (thoracic cardiac nerves), whence they converge on the cardiac plexus before reaching the heart. Parasympathetic fibers derived from CN X and its recurrent laryngeal branch decrease heart rate and stroke volume through release of acetylcholine, principally in the vicinity of the sinuatrial node.

The rectus sheath is formed by the aponeuroses of the abdominal wall musculature. Between the costal margin and the umbilicus, the aponeurosis of the internal oblique muscle splits; one portion passes anterior and the other posterior to the rectus abdominis muscle. The aponeurosis of the external oblique muscle fuses with the anterior leaflet of the aponeurosis of the internal oblique muscle to form the anterior wall of the rectus sheath. The aponeurosis of the transversus abdominis muscle fuses with the posterior leaflet of the aponeurosis of the internal oblique muscle to form the posterior wall of the rectus sheath. Approximately midway between the umbilicus and symphysis pubis, the aponeuroses of the internal oblique and transversus abdominis muscles pass anterior to the rectus abdominis muscle to contribute to the anterior leaf of the rectus sheath. This abrupt transition results in a free edge to the posterior rectus sheath, known as the arcuate line (of Douglas). Between this line and the pubis, only the transversalis fascia separates the rectus abdominis muscle from the peritoneum. It is here, where the inferior epigastric artery gains access to the rectus sheath, that ventral lateral (spigelian) herniation may occur.

The ilioinguinal nerve exits the abdominal wall through the superficial inguinal ring, where it is applied to the anterior surface of the spermatic cord. Section of this nerve will result in paresthesia over the base of the penis and scrotum. The femoral branch of the genitofemoral nerve innervates the upper medial surface of the thigh, where it mediates the afferent limb of the cremasteric reflex. The efferent limb of this reflex is carried by the genital branch of the genitofemoral nerve, which lies within the cremaster layer. The dartos response, which is sympathetic, arises from the sacral sympathetic chain and reaches the pudendal nerve via gray rami communicantes.

The uterus is formed by fusion of the paired paramesonephric ducts. The uterine tubes are the unfused portions of these ducts. Patency of the uterine tubes may be ascertained by hysterosalpingography, wherein radiopaque material is injected into the uterine cavity and uterine tubes through a catheter inserted into the external cervical os. Radiographs delineate the cavity of the body of the uterus and the uterine tubes. Spillage of the contrast material through the abdominal ostia into the peritoneal cavity demonstrates normal patency of the uterine tubes. The abdominal ostia of the uterine tubes permit passage of infection, air, and spermatozoa into the female peritoneal cavity. The rare rectouterine fistula would result in the appearance of contrast media in the rectum. A vesicovaginal fistula between the vagina and urethra or bladder would not be evident on a hysterosalpingogram.

A fall on the extended hand will frequently dislocate the lunate bone anteriorly. This dislocated bone may then impinge on the tendons of the extrinsic digital flexor muscles and thereby prevent flexion of the fingers. Compression of the median nerve in the carpal tunnel cannot explain this observation because the prime flexors of the digits are the extrinsic flexors (flexors digitorum superficialis and profundus), which receive their innervation in the forearm, well proximal to the injury. The dorsal interossei, innervated by the ulnar nerve, are digital extensors. The recurrent branch of the median nerve innervates the thenar muscles.

The greater superficial petrosal nerve leaves the facial nerve (CN VII) at the geniculate ganglion. It carries secretomotor neurons from the superior salivatory nucleus to the pterygopalatine ganglion and joins along the way with the sympathetic deep petrosal nerve to become the nerve of the pterygoid canal.

Because few intrahepatic arterial anastomoses exist, ligation of a left or right hepatic artery or of an aberrant (accessory) hepatic artery will result in ischemic necrosis of the region of the liver supplied by that vessel. The left hepatic artery supplies the left lobe and the quadrate lobe, as well as half the caudate lobe. The right hepatic artery supplies the right lobe and the other half of the caudate lobe; it also usually supplies the gallbladder through the cystic artery. No major extrahepatic anastomotic connections distal to the right gastric artery exist, and the hepatic portal vein has far too low a partial pressure of oxygen to supply the metabolic requirements of liver parenchyma.

Each of the muscles listed above is innervated by the deep branch of the radial nerve or its terminal portion, the posterior interosseus nerve. The deep radial nerve passes between the deep and superficial layers of the supinator muscle and lies on a bare area of the radius where it may be compressed by action of the supinator or damaged by a fracture of the radius.

The horizontal direction of the fibers of the clavicular head of the pectoralis major muscle draws the humerus medially and causes the distal fragment of the bone to sublux. The sternal head of this muscle also has the effect of pulling the arm medially, an effect that is normally offset by the strutlike action of the clavicle.

The saphenous nerve accompanies the great saphenous vein along the medial aspect of the leg and foot as far as the great toe. The superficial fibular nerve innervates the central portion of the dorsum of the foot. The sural cutaneous nerve innervates the lateral aspect of the foot. The medial and lateral plantar branches of the tibial nerve supply the sole of the foot.

Visceral afferent pain fibers from the gallbladder travel through the celiac plexus, thence along the greater splanchnic nerves to levels T5–T9 of the spinal cord. Thus, pain originating from the gallbladder will be referred to (appear as if coming from) the dermatomes served by T5–T9, which include a band from the infrascapular region to the epigastrium.

The transverse midplane diameter is measured between the ischial spines. It can be approximated by the somewhat greater transverse diameter measured between the ischial tuberosities. The distance from the lower margin of the pubic symphysis to the sacroiliac joint defines the sagittal diameter, which is usually the greatest dimension and, therefore, unimportant.

To unscrew a knee from its locked and slightly hyperextended position, the popliteus muscle contracts and causes medial rotation of the tibia or, if the foot is planted, lateral rotation of the femur. This movement frees the medial femoral condyle from its posterior position on the tibial condylar surface. The quadriceps femoris then relaxes, and knee flexion occurs by contraction of the hamstring muscles, assisted by the short head of the biceps femoris, sartorius, gracilis, and gastrocnemius muscles.

Demyelinization would most profoundly affect the function of well-myelinated large neurons of the lateral corticospinal tract, as well as the lower motor neurons originating in the ventral horn of the spinal cord. However, because reflex pathways were intact and hyperactive, the disease process must be confined to the upper motor neurons of the corticospinal tract. The lateral corticospinal tract carries messages from the contralateral motor cortex to lower motor neurons in the ventral horn of the spinal cord. Loss of this pathway results in spastic, rather than flaccid, paralysis. The other tracts (vestibulo-, rubro-, and tecto-spinal) are composed of fine, sparsely myelinated neurons and would be less affected by the anemia.

Compressing the hepatic pedicle and its contained vascular structures between the forefinger placed in the omental foramen (of Winslow) and the thumb placed anteriorly is a convenient way to stem extrahepatic hemorrhage until the source of bleeding can be located and ligated. The blood supply to the liver is variable; several potential anastomotic loops exist between branches of the extrahepatic arterial system. Thus, ligation of the common hepatic artery proximal to the gastroduodenal artery will enable arterial blood to reach the liver from branches of the splenic artery (via anastomotic left and right gastroepiploic arteries) and the superior mesenteric artery (via the anastomotic inferior and superior pancreaticoduodenal arteries). Ligation of the proper hepatic artery proximal to the origin of the right gastric artery will enable arterial blood to reach the liver from branches of the celiac artery (via anastomotic left and right gastric arteries). However, ligation distal to the juncture of the right gastric artery will terminate most of, if not all, the blood supply to the liver and incur a danger of ischemia, if not necrosis, of hepatic tissue. Because accessory or aberrant hepatic arteries usually are not sources of collateral blood supply to the liver, they cannot be relied on to provide intrahepatic anastomotic connections.

The medial meniscus is attached to the medial collateral ligament. It is relatively immovable and, therefore, unable to evade damage such as occurred in this case. The medial meniscus is clearly not attached to the popliteus muscle or to the anterior cruciate ligament.

Contraction of the intercostal muscles causes rotation of the costovertebral joints and elevation of the sternal ends of the upper (2–6) ribs. Along with slight movement of the sternomanubrial joint, particularly in the young, this "pump-handle movement" increases the anteroposterior (AP) diameter of the chest. The transverse diameter of the thoracic cavity increases when contraction of the intercostal muscles also elevates the midportion of the ribs (bucket-handle movement). Contraction of the diaphragm increases the vertical diameter of the thoracic cavity.

The common fibular (peroneal) nerve bifurcates into superficial and deep branches. The deep fibular nerve innervates all muscles of the anterior compartment of the leg. The superficial fibular nerve emerges from the deep fascia and descends in the lateral compartment, where it innervates the peroneus longus and brevis muscles before dividing into median dorsal cutaneous and intermediate dorsal cutaneous nerves, which supply the distal third of the leg, dorsum of the foot, and all the toes. The saphenous nerve (the terminal branch of the common femoral nerve) distributes cutaneous branches to the anterior and medial aspects of the leg as well as to the dorsomedial aspect of the foot. The sural nerve follows the course of the lesser saphenous vein and becomes the lateral sural cutaneous nerve to supply the anterolateral aspect of the foot.

The deep circumflex iliac artery, which arises from the internal iliac artery opposite the inferior epigastric artery, parallels the inguinal ligament as it courses toward the anterosuperior iliac spine. The external pudendal and superficial epigastric arteries are branches of the femoral artery that supply, respectively, the superficial pubic (hypogastric) region, the inguinal regions, and the anterior surfaces of the scrotum or labia majora. The inferior epigastric artery, a branch of the external iliac artery, courses superomedially beneath the aponeuroses of the abdominal wall to gain access to the rectus sheath by passing anterior to the arcuate line (of Douglas). An aberrant obturator artery (present in about 30% of the population) usually arises from the inferior epigastric artery and courses inferiorly deep to the pubic ramus to the obturator foramen.

Afferent innervation from the heart and coronary arteries travels to the cardiac plexus along the sympathetic pathways. Once the afferent fibers pass through the cardiac plexus, they run along the cervical and thoracic cardiac nerves to the cervical and upper four thoracic sympathetic ganglia. Having traversed these ganglia, the fibers gain access (via the white rami communicantes) to the upper four thoracic spinal nerves and the corresponding levels of the spinal cord. The visceral afferent fibers associated with the vagus nerve are associated with reflexes and do not carry nociceptive information. The greater, lesser, and least splanchnic nerves convey visceral afferents from the abdominal region.

The visceral afferent fibers that mediate sensation from the fundus and body of the uterus, as well as from the oviducts, tend to travel along the sympathetic nerve pathways (via the hypogastric nerve and lumbar splanchnics) to reach the upper lumbar levels (L1–L2) of the spinal cord. Thus, uterine pain will be referred to (appear as if originating from) the upper lumbar dermatomes and produce apparent backache. The visceral afferent fibers that mediate sensation from the cervical neck of the uterus travel along the parasympathetic pathways (via the pelvic splanchnic nerves [nervi erigentes]) to the midsacral levels (S2–S4) of the spinal cord. In this instance, pain originating from the cervix will be referred to the midsacral dermatomes and produce pain that appears to arise from the perineum, gluteal region, and legs.