Abdomen - Overview and Surface Anatomy PDF

CONCEPTUAL OVERVIEW  The abdomen is a roughly cylindrical chamber extending from the inferior margin of the thorax to the superior margin of the pelvis and the lower limb (Fig. 4.1A). The inferior thoracic aperture forms the superior opening to the abdomen and is closed by the diaphrag...

CONCEPTUAL OVERVIEW  The abdomen is a roughly cylindrical chamber extending from the inferior margin of the thorax to the superior margin of the pelvis and the lower limb (Fig. 4.1A). The inferior thoracic aperture forms the superior opening to the abdomen and is closed by the diaphragm. Inferiorly, the deep abdominal wall is continuous with the pelvic wall at the pelvic inlet. Superficially, the inferior limit of the abdominal wall is the superior margin of the lower limb. The chamber enclosed by the abdominal wall contains a single large peritoneal cavity, which freely communicates with the pelvic cavity.  Abdominal viscera are either suspended in the peritoneal cavity by mesenteries or positioned between the cavity and the musculoskeletal wall (Fig. 4.1B). Abdominal viscera include: major elements of the gastrointestinal system—the caudal end of the esophagus, stomach, small and large intestines, liver, pancreas, and gallbladder; the spleen; components of the urinary system—kidneys and ureters; the suprarenal glands; and major neurovascular structures. FUNCTIONS HOUSES AND PROTECTS MAJOR VISCERA The abdomen houses major elements of the gastrointestinal system (Fig. 4.2), the spleen, and parts of the urinary system. Much of the liver, gallbladder, stomach, and spleen and parts of the colon are under the domes of the diaphragm, which project superiorly above the costal margin of the thoracic wall, and as a result these abdominal viscera are protected by the thoracic wall. The superior poles of the kidneys are deep to the lower ribs. Viscera not under the domes of the diaphragm are sup- ported and protected predominantly by the muscular walls of the abdomen.  BREATHING One of the most important roles of the abdominal wall is to assist in breathing: It relaxes during inspiration to accommodate expansion of the thoracic cavity and the inferior displacement of abdominal viscera during contraction of the diaphragm (Fig. 4.3). During expiration, it contracts to assist in elevating the domes of the diaphragm, thus reducing thoracic volume. Material can be expelled from the airway by forced expiration using the abdominal muscles, as in coughing or sneezing.  CHANGES IN INTRAABDOMINAL PRESSURE Contraction of abdominal wall muscles can dramatically increase intraabdominal pressure when the diaphragm is in a fixed position (Fig. 4.4). Air is retained in the lungs by closing valves in the larynx in the neck. Increased intraabdominal pressure assists in voiding the contents of the bladder and rectum and in giving birth. COMPONENT PARTS  WALL The abdominal wall consists partly of bone but mainly of muscle (Fig. 4.5). The skeletal elements of the wall (Fig. 4.5A) are: the five lumbar vertebrae and their intervening intervertebral discs, the superior expanded parts of the pelvic bones, and bony components of the inferior thoracic wall, includ- ing the costal margin, rib XII, the end of rib XI, and the xiphoid process.  WALL Muscles make up the rest of the abdominal wall (Fig. 4.5B): Lateral to the vertebral column, the quadratus lumbo- rum, psoas major, and iliacus muscles reinforce the posterior aspect of the wall. The distal ends of the psoas major and iliacus muscles pass into the thigh and are major flexors of the hip joint. Lateral parts of the abdominal wall are predominantly formed by three layers of muscles, which are similar in orientation to the intercostal muscles of the thorax— transversus abdominis, internal oblique, and external oblique. Anteriorly, a segmented muscle (the rectus abdominis) on each side spans the distance between the inferior thoracic wall and the pelvis. Structural continuity between posterior, lateral, and anterior parts of the abdominal wall is provided by thick fascia posteriorly and by flat tendinous sheets (aponeuroses) derived from muscles of the lateral wall. A fascial layer of varying thickness separates the abdominal wall from the peritoneum, which lines the abdominal cavity. ABDOMINAL CAVITY The general organization of the abdominal cavity is one in which a central gut tube (gastrointestinal system) is suspended from the posterior abdominal wall and partly from the anterior abdominal wall by thin sheets of tissue (mesenteries; Fig. 4.6): a ventral (anterior) mesentery for proximal regions of the gut tube; a dorsal (posterior) mesentery along the entire length of the system. 1. Different parts of these two mesenteries are named according to the organs they suspend or with which they are associated. 2. Major viscera, such as the kidneys, that are not sus- pended in the abdominal cavity by mesenteries are associated with the abdominal wall. 3. The abdominal cavity is lined by peritoneum, which consists of an epithelial-like single layer of cells (the mesothelium) together with a supportive layer of connective tissue. Peritoneum is similar to the pleura and serous pericardium in the thorax. 4. The peritoneum reflects off the abdominal wall to become a component of the mesenteries that suspend the viscera. 1. Parietal peritoneum lines the abdominal wall. 2. Visceral peritoneum covers suspended organs. L Normally, elements of the gastrointestinal tract and its derivatives completely fill the abdominal cavity, making the peritoneal cavity a potential space, and visceral peritoneum on organs and parietal peritoneum on the adjacent abdominal wall slide freely against one another. Abdominal viscera are either intraperitoneal or retroperitoneal: Intraperitoneal structures, such as elements of the gastrointestinal system, are suspended from the abdominal wall by mesenteries; Structures that are not suspended in the abdominal cavity by a mesentery and that lie between the parietal peritoneum and abdominal wall are retroperitoneal in position. Retroperitoneal structures include the kidneys and ureters, which develop in the region between the peritoneum and the abdominal wall and remain in this position in the adult.  During development, some organs, such as parts of the small and large intestines, are suspended initially in the abdominal cavity by a mesentery, and later become retro- peritoneal secondarily by fusing with the abdominal wall (Fig. 4.7).  Large vessels, nerves, and lymphatics are associated with the posterior abdominal wall along the median axis of the body in the region where, during development, the peritoneum reflects off the wall as the dorsal mesentery, which supports the developing gut tube. As a consequence, branches of the neurovascular structures that pass to parts of the gastrointestinal system are unpaired, originate from the anterior aspects of their parent structures, and travel in mesenteries or pass retroperitoneally in areas where the mesenteries secondarily fuse to the wall. Generally, vessels, nerves, and lymphatics to the abdominal wall and to organs that originate as retroperitoneal structures branch laterally from the central neurovascular structures and are usually paired, one on each side. INFERIOR THORACIC APERTURE The superior aperture of the abdomen is the inferior thoracic aperture, which is closed by the diaphragm. The margin of the inferior thoracic aperture consists of vertebra TXII, rib XII, the distal end of rib XI, the costal margin, and the xiphoid process of the sternum.  DIAPHRAGM The musculotendinous diaphragm separates the abdomen from the thorax. The diaphragm attaches to the margin of the inferior thoracic aperture, but the attachment is complex posteri- orly and extends into the lumbar area of the vertebral column (Fig. 4.8). On each side, a muscular extension (crus) firmly anchors the diaphragm to the anterolateral surface of the vertebral column as far down as vertebra LIII on the right and vertebra LII on the left.  Because the costal margin is not complete posteriorly, the diaphragm is anchored to arch-shaped (arcuate) ligaments, which span the distance between available bony points and the intervening soft tissues: Medial and lateral arcuate ligaments cross muscles of the posterior abdominal wall and attach to vertebrae, the transverse processes of vertebra LI and rib XII, respectively. A median arcuate ligament crosses the aorta and is continuous with the crus on each side. The posterior attachment of the diaphragm extends much farther inferiorly than the anterior attachment. Consequently, the diaphragm is an important component of the posterior abdominal wall, to which a number of viscera are related.  PELVIC INLET 1. The abdominal wall is continuous with the pelvic wall at the pelvic inlet, and the abdominal cavity is continuous with the pelvic cavity. 2. The circular margin of the pelvic inlet is formed entirely by bone: posteriorly by the sacrum, anteriorly by the pubic symphysis, and laterally, on each side, by a distinct bony rim on the pelvic bone (Fig. 4.9).  Because of the way in which the sacrum and attached pelvic bones are angled posteriorly on the vertebral column, the pelvic cavity is not oriented in the same vertical plane as the abdominal cavity. Instead, the pelvic cavity projects posteriorly, and the inlet opens anteriorly and somewhat superiorly (Fig. 4.10). RELATIONSHIP TO OTHER REGIONS  Thorax The abdomen is separated from the thorax by the dia- phragm. Structures pass between the two regions through or posterior to the diaphragm (see Fig. 4.8). Pelvis The pelvic inlet opens directly into the abdomen and struc- tures pass between the abdomen and pelvis through it. The peritoneum lining the abdominal cavity is continuous with the peritoneum in the pelvis. Consequently, the abdominal cavity is entirely continuous with the pelvic cavity (Fig. 4.11). Infections in one region can therefore freely spread into the other. The bladder expands superiorly from the pelvic cavity into the abdominal cavity and, during pregnancy, the uterus expands freely superiorly out of the pelvic cavity into the abdominal cavity.  Lower limb The abdomen communicates directly with the thigh through an aperture formed anteriorly between the inferior margin of the abdominal wall (marked by the inguinal ligament) and the pelvic bone (Fig. 4.12). Structures that pass through this aperture are: the major artery and vein of the lower limb; the femoral nerve, which innervates the quadriceps femoris muscle, which extends the knee; lymphatics; and the distal ends of psoas major and iliacus muscles, which flex the thigh at the hip joint. As vessels pass inferior to the inguinal ligament, their names change—the external iliac artery and vein of the abdomen become the femoral artery and vein of the thigh. KEY FEATURES ARRANGEMENT OF ABDOMINAL VISCERA IN THE ADULT A basic knowledge of the development of the gastro- intestinal tract is needed to understand the arrangement of viscera and mesenteries in the abdomen (Fig. 4.13). The early gastrointestinal tract is oriented longitudinally in the body cavity and is suspended from surrounding walls by a large dorsal mesentery and a much smaller ventral mesentery. Superiorly, the dorsal and ventral mesenteries are anchored to the diaphragm. The primitive gut tube consists of the foregut, the midgut, and the hindgut. Massive longitudinal growth of the gut tube, rotation of selected parts of the tube, and secondary fusion of some viscera and their associated mesenteries to the body wall participate in generating the adult arrangement of abdominal organs. DEVELOPMENT OF THE FOREGUT In abdominal regions, the foregut gives rise to the distal end of the esophagus, the stomach, and the proximal part of the duodenum. The foregut is the only part of the gut tube suspended from the wall by both the ventral and dorsal mesenteries. A diverticulum from the anterior aspect of the foregut grows into the ventral mesentery, giving rise to the liver and gallbladder, and, ultimately, to the ventral part of the pancreas. DEVELOPMENT OF THE FOREGUT The dorsal part of the pancreas develops from an out- growth of the foregut into the dorsal mesentery. The spleen develops in the dorsal mesentery in the region between the body wall and presumptive stomach. In the foregut, the developing stomach rotates clockwise and the associated dorsal mesentery, containing the spleen, moves to the left and greatly expands. During this process, part of the mesentery becomes associated with, and secondarily fuses with, the left side of the body wall. At the same time, the duodenum, together with its dorsal mesentery and an appreciable part of the pancreas, swings to the right and fuses to the body wall. DEVELOPMENT OF THE FOREGUT Secondary fusion of the duodenum to the body wall, massive growth of the liver in the ventral mesentery, and fusion of the superior surface of the liver to the diaphragm restrict the opening to the space enclosed by the ballooned dorsal mesentery associated with the stomach. This restricted opening is the omental foramen (epiploic foramen). The part of the abdominal cavity enclosed by the expanded dorsal mesentery, and posterior to the stomach, is the omental bursa (lesser sac). Access, through the omental foramen, to this space from the rest of the perito- neal cavity (greater sac) is inferior to the free edge of the ventral mesentery. DEVELOPMENT OF THE FOREGUT Part of the dorsal mesentery that initially forms part of the lesser sac greatly enlarges in an inferior direction, and the two opposing surfaces of the mesentery fuse to form an apron-like structure (the greater omentum). The greater omentum is suspended from the greater curvature of the stomach, lies over other viscera in the abdominal cavity, and is the first structure observed when the abdominal cavity is opened anteriorly. DEVELOPMENT OF THE MIDGUT The midgut develops into the distal part of the duodenum and the jejunum, ileum, ascending colon, and proximal two-thirds of the transverse colon. A small yolk sac pro- jects anteriorly from the developing midgut into the umbilicus. Rapid growth of the gastrointestinal system results in a loop of the midgut herniating out of the abdominal cavity and into the umbilical cord. As the body grows in size and the connection with the yolk sac is lost, the midgut returns to the abdominal cavity. DEVELOPMENT OF THE MIDGUT While this process is occurring, the two limbs of the midgut loop rotate counterclockwise around their combined central axis, and the part of the loop that becomes the cecum descends into the inferior right aspect of the cavity. The superior mesenteric artery, which supplies the midgut, is at the center of the axis of rotation. The cecum remains intraperitoneal, the ascending colon fuses with the body wall becoming secondarily retroperitoneal, and the transverse colon remains suspended by its dorsal mesentery (transverse mesocolon). The greater omentum hangs over the transverse colon and the mesocolon and usually fuses with these structures.  DEVELOPMENT OF THE HINDGUT The distal one-third of the transverse colon, descending colon, sigmoid colon, and superior part of the rectum develop from the hindgut. Proximal parts of the hindgut swing to the left and become the descending colon and sigmoid colon. The descending colon and its dorsal mesentery fuse to the body wall, while the sigmoid colon remains intraperitoneal. The sigmoid colon passes through the pelvic inlet and is con- tinuous with the rectum at the level of vertebra SIII.  SKIN AND MUSCLES OF THE ANTERIOR AND LATERAL ABDOMINAL WALL AND THORACIC INTERCOSTAL NERVES The anterior rami of thoracic spinal nerves T7 to T12 follow the inferior slope of the lateral parts of the ribs and cross the costal margin to enter the abdominal wall (Fig. 4.14). Intercostal nerves T7 to T11 supply skin and muscle of the abdominal wall, as does the subcostal nerve T12. In addition, T5 and T6 supply upper parts of the external oblique muscle of the abdominal wall; T6 also supplies cutaneous innervation to skin over the xiphoid. Skin and muscle in the inguinal and suprapubic regions of the abdominal wall are innervated by L1 and not by thoracic nerves. Dermatomes of the anterior abdominal wall are indicated in Figure 4.14. In the midline, skin over the infrasternal angle is T6 and that around the umbilicus is T10. L1 innervates skin in the inguinal and suprapubic regions. Muscles of the abdominal wall are innervated segmentally in patterns that generally reflect the patterns of the overlying dermatomes. THE GROIN IS A WEAK AREA IN THE ANTERIOR ABDOMINAL WALL During development, the gonads in both sexes descend from their sites of origin on the posterior abdominal wall into the pelvic cavity in women and the developing scrotum in men (Fig. 4.15). In both men and women, the groin (inguinal region) is a weak area in the abdominal wall (Fig. 4.15) and is the site of inguinal hernias. THE GROIN IS A WEAK AREA IN THE ANTERIOR ABDOMINAL WALL Before descent, a cord of tissue (the gubernaculum) passes through the anterior abdominal wall and connects the inferior pole of each gonad with primordia of the scrotum in men and the labia majora in women (labioscrotal swellings). A tubular extension (the processus vaginalis) of the peritoneal cavity and the accompanying muscular layers of the anterior abdominal wall project along the gubernaculum on each side into the labioscrotal swellings. In men, the testis, together with its neurovascular structures and its efferent duct (the ductus deferens) descends into the scrotum along a path, initially defined by the gubernaculum, between the processus vaginalis and the accompanying coverings derived from the abdominal wall. THE GROIN IS A WEAK AREA IN THE ANTERIOR ABDOMINAL WALL All that remains of the gubernaculum is a connective tissue remnant that attaches the caudal pole of the testis to the scrotum. The inguinal canal is the passage through the anterior abdominal wall created by the processus vaginalis. The spermatic cord is the tubular extension of the layers of the abdominal wall into the scrotum that contains all structures passing between the testis and the abdomen. The distal sac-like terminal end of the spermatic cord on each side contains the testis, associated structures, and the now isolated part of the peritoneal cavity (the cavity of the tunica vaginalis). In women, the gonads descend to a position just inside the pelvic cavity and never pass through the anterior abdominal wall. As a result, the only major structure passing through the inguinal canal is a derivative of the gubernaculum (the round ligament of the uterus). VERTEBRAL LEVEL LI The transpyloric plane is a horizontal plane that transects the body through the lower aspect of vertebra LI (Fig. 4.16). It: is about midway between the jugular notch and the pubic symphysis, and crosses the costal margin on each side at roughly the ninth costal cartilage;  VERTEBRAL LEVEL LI crosses through the opening of the stomach into the duodenum (the pyloric orifice), which is just to the right of the body of LI; the duodenum then makes a characteristic C-shaped loop on the posterior abdominal wall and crosses the midline to open into the jejunum just to the left of the body of vertebra LII, whereas the head of the pancreas is enclosed by the loop of the duodenum, and the body of the pancreas extends across the midline to the left; crosses through the body of the pancreas; and approximates the position of the hila of the kidneys; though because the left kidney is slightly higher than the right, the transpyloric plane crosses through the inferior aspect of the left hilum and the superior part of the right hilum.  THE GASTROINTESTINAL SYSTEM AND ITS DERIVATIVES ARE SUPPLIED BY THREE MAJOR ARTERIES Three large unpaired arteries branch from the anterior surface of the abdominal aorta to supply the abdominal part of the gastrointestinal tract and all of the structures (liver, pancreas, and gallbladder) to which this part of the gut gives rise during development (Fig. 4.17). These arteries pass through derivatives of the dorsal and ventral mesenteries to reach the target viscera. These vessels therefore also supply structures such as the spleen and lymph nodes that develop in the mesenteries. THE GASTROINTESTINAL SYSTEM AND ITS DERIVATIVES ARE SUPPLIED BY THREE MAJOR ARTERIES 1. These three arteries are: the celiac artery, which branches from the abdominal aorta at the upper border of vertebra LI and supplies the foregut; the superior mesenteric artery, which arises from the abdominal aorta at the lower border of vertebra LI and supplies the midgut; and the inferior mesenteric artery, which branches from the abdominal aorta at approximately vertebral level LIII and supplies the hindgut. VENOUS SHUNTS FROM LEFT TO RIGHT All blood returning to the heart from regions of the body other than the lungs flows into the right atrium of the heart. The inferior vena cava is the major systemic vein in the abdomen and drains this region together with the pelvis, perineum, and both lower limbs (Fig. 4.18).  VENOUS SHUNTS FROM LEFT TO RIGHT The inferior vena cava lies to the right of the vertebral column and penetrates the central tendon of the diaphragm at approximately vertebral level TVIII. A number of large vessels cross the midline to deliver blood from the left side of the body to the inferior vena cava. One of the most significant is the left renal vein, which drains the kidney, suprarenal gland, and gonad on the same side. Another is the left common iliac vein, which crosses the midline at approximately vertebral level LV to join with its partner on the right to form the inferior vena cava. These veins drain the lower limbs, the pelvis, the perineum, and parts of the abdominal wall. Other vessels crossing the midline include the left lumbar veins, which drain the back and posterior abdominal wall on the left side. ALL VENOUS DRAINAGE FROM THE GASTROINTESTINAL SYSTEM PASSES THROUGH THE LIVER Blood from abdominal parts of the gastrointestinal system and the spleen passes through a second vascular bed, in the liver, before ultimately returning to the heart (Fig. 4.19). ALL VENOUS DRAINAGE FROM THE GASTROINTESTINAL SYSTEM PASSES THROUGH THE LIVER Venous blood from the digestive tract, pancreas, gall- bladder, and spleen enters the inferior surface of the liver through the large hepatic portal vein. This vein then ramifies like an artery to distribute blood to small endothelial-lined hepatic sinusoids, which form the vascular exchange network of the liver. After passing through the sinusoids, the blood collects in a number of short hepatic veins, which drain into the inferior vena cava just before the inferior vena cava penetrates the diaphragm and enters the right atrium of the heart. Normally, vascular beds drained by the hepatic portal system interconnect, through small veins, with beds drained by systemic vessels, which ultimately connect directly with either the superior or inferior vena cava. PORTACAVAL ANASTOMOSES Among the clinically most important regions of overlap between the portal and caval systems are those at each end of the abdominal part of the gastrointestinal system: around the inferior end of the esophagus; around the inferior part of the rectum.  Small veins that accompany the degenerate umbilical vein (round ligament of the liver) establish another important portacaval anastomosis. The round ligament of the liver connects the umbilicus of the anterior abdominal wall with the left branch of the portal vein as it enters the liver. The small veins that accompany this ligament form a connection between the portal system and paraumbilical regions of the abdominal wall, which drain into systemic veins.  Other regions where portal and caval systems interconnect include: where the liver is in direct contact with the diaphragm (the bare area of the liver); where the wall of the gastrointestinal tract is in direct contact with the posterior abdominal wall (retroperitoneal areas of the large and small intestine); and the posterior surface of the pancreas (much of the pancreas is secondarily retroperitoneal). BLOCKAGE OF THE HEPATIC PORTAL VEIN OR OF VASCULAR CHANNELS IN THE LIVER Blockage of the hepatic portal vein or of vascular channels in the liver can affect the pattern of venous return from abdominal parts of the gastrointestinal system. Vessels that interconnect the portal and caval systems can become greatly enlarged and tortuous, allowing blood in tributaries of the portal system to bypass the liver, enter the caval system, and thereby return to the heart. Portal hypertension can result in esophageal and rectal varices and in caput medusae in which systemic vessels that radiate from para-umbilical veins enlarge and become visible on the abdominal wall. ABDOMINAL VISCERA ARE SUPPLIED BY A LARGE PREVERTEBRAL PLEXUS Innervation of the abdominal viscera is derived from a large prevertebral plexus associated mainly with the anterior and lateral surfaces of the aorta (Fig. 4.20). Branches are distributed to target tissues along vessels that originate from the abdominal aorta. ABDOMINAL VISCERA ARE SUPPLIED BY A LARGE PREVERTEBRAL PLEXUS The prevertebral plexus contains sympathetic, para- sympathetic, and visceral sensory components: Sympathetic components originate from spinal cord levels T5 to L2. Parasympathetic components are from the vagus nerve [X] and spinal cord levels S2 to S4. Visceral sensory fibers generally parallel the motor pathways. SURFACE ANATOMY  Surface anatomy of the pelvis and perineum Palpable bony features of the pelvis are used as landmarks for: locating soft tissue structures, visualizing the orientation of the pelvic inlet, and defining the margins of the perineum. The ability to recognize the normal appearance of structures in the perineum is an essential part of a physical examination. In women, the cervix can be visualized directly by opening the vaginal canal using a speculum. In men, the size and texture of the prostate in the pelvic cavity can be assessed by digital palpation through the anal aperture. 1. Orientation of the pelvis and perineum in the anatomical position 2. In the anatomical position, the anterior superior iliac spines and the anterior superior edge of the pubic symphysis lie in the same vertical plane. The pelvic inlet faces anterosuperiorly. The urogenital triangle of the perineum is oriented in an almost horizontal plane and faces inferiorly, whereas the anal triangle is more vertical and faces posteriorly (Figs. 5.80 and 5.81). 1. Orientation of the pelvis and perineum in the anatomical position 2. In the anatomical position, the anterior superior iliac spines and the anterior superior edge of the pubic symphysis lie in the same vertical plane. The pelvic inlet faces anterosuperiorly. The urogenital triangle of the perineum is oriented in an almost horizontal plane and faces inferiorly, whereas the anal triangle is more vertical and faces posteriorly (Figs. 5.80 and 5.81).  How to define the margins of the perineum The pubic symphysis, ischial tuberosities, and tip of the sacrum are palpable on patients and can be used to define the boundaries of the perineum. This is best done with patients lying on their backs with their thighs flexed and abducted in the lithotomy position (Fig. 5.82).  The ischial tuberosities are palpable on each side as large bony masses near the crease of skin (gluteal fold) between the thigh and gluteal region. They mark the lateral corners of the diamond-shaped perineum. The tip of the coccyx is palpable in the midline posterior to the anal aperture and marks the most posterior limit of the perineum. The anterior limit of the perineum is the pubic symphy- sis. In women, this is palpable in the midline deep to the mons pubis. In men, the pubic symphysis is palpable immediately superior to where the body of the penis joins the lower abdominal wall. Imaginary lines that join the ischial tuberosities with the pubic symphysis in front, and with the tip of the coccyx behind, outline the diamond-shaped perineum. An addi- tional line between the ischial tuberosities divides the perineum into two triangles, the urogenital triangle ante- riorly and anal triangle posteriorly. This line also approxi- mates the position of the posterior margin of the perineal membrane. The midpoint of this line marks the location of the perineal body or central tendon of the perineum.  IDENTIFICATION OF STRUCTURES IN THE ANAL TRIANGLE The anal triangle is the posterior half of the perineum. The base of the triangle faces anteriorly and is an imaginary line joining the two ischial tuberosities. The apex of the triangle is the tip of the coccyx; the lateral margins can be approximated by lines joining the coccyx to the ischial tuberosities. In both women and men, the major feature of the anal triangle is the anal aperture in the center of the triangle. Fat fills the ischio-anal fossa on each side of the anal aperture (Fig. 5.83). IDENTIFICATION OF STRUCTURES IN THE UROGENITAL TRIANGLE OF WOMEN Identification of structures in the anal triangle The anal triangle is the posterior half of the perineum. The base of the triangle faces anteriorly and is an imaginary line joining the two ischial tuberosities. The apex of the triangle is the tip of the coccyx; the lateral margins can be approximated by lines joining the coccyx to the ischial tuberosities. In both women and men, the major feature of the anal triangle is the anal aperture in the center of the triangle. Fat fills the ischio-anal fossa on each side of the anal aperture (Fig. 5.83).  IDENTIFICATION OF STRUCTURES IN THE UROGENITAL TRIANGLE OF WOMEN The urogenital triangle is the anterior half of the perineum. The base of the triangle faces posteriorly and is an imagi- nary line joining the two ischial tuberosities. The apex of the triangle is the pubic symphysis. The lateral margins can be approximated by lines joining the pubic symphysis to the ischial tuberosities. These lines overlie the ischiopubic rami, which can be felt on deep palpation. In women, the major contents of the urogenital triangle are the clitoris, the vestibule, and skin folds that together form the vulva (Fig. 5.84A,B).  Two thin skin folds, the labia minora, enclose between them a space termed the vestibule into which the vagina and the urethra open (Fig. 5.84C). Gentle lateral traction on the labia minora opens the vestibule and reveals a soft tissue mound on which the urethra opens. The para- urethral (Skene’s) glands, one on each side, open into the skin crease between the urethra and the labia minora (Fig. 5.84D).  Posterior to the urethra is the vaginal opening. The vaginal opening (introitus) is ringed by remnants of the hymen that originally closes the vaginal orifice and is usually ruptured during the first sexual intercourse. The ducts of the greater vestibular (Bartholin’s) glands, one on each side, open into the skin crease between the hymen and the adjacent labium minus (Fig. 5.84D).  The labia minora each bifurcate anteriorly into medial and lateral folds. The medial folds unite at the midline to form the frenulum of the clitoris. The larger lateral folds also unite across the midline to form the clitoral hood or prepuce that covers the glans clitoris and distal parts of the body of the clitoris. Posterior to the vaginal orifice, the labia minora join, forming a transverse skin fold (the fourchette). The labia majora are broad folds positioned lateral to the labia minora. They come together in front to form the mons pubis, which overlies the inferior aspect of the pubic sym- physis. The posterior ends of the labia majora are separated by a depression termed the posterior commissure, which overlies the position of the perineal body.  The cervix is visible when the vaginal canal is opened with a speculum (Fig. 5.84E). The external cervical os opens onto the surface of the dome-shaped cervix. A recess or gutter, termed the fornix, occurs between the cervix and the vaginal wall and is further subdivided, based on loca- tion, into anterior, posterior, and lateral fornices. The roots of the clitoris occur deep to surface features of the perineum and are attached to the ischiopubic rami and the perineal membrane.  The bulbs of the vestibule (Fig. 5.84F), composed of erectile tissues, lie deep to the labia minora on either side of the vestibule. These erectile masses are continuous, via thin bands of erectile tissues, with the glans clitoris, which is visible under the clitoral hood. The greater vestibular glands occur posterior to the bulbs of the vestibule on either side of the vaginal orifice.  The crura of the clitoris are attached, one on each side, to the ischiopubic rami. Each crus is formed by the attached part of the corpus cavernosum. Anteriorly, these erectile corpora detach from bone, curve posteroinferiorly, and unite to form the body of the clitoris. The body of the clitoris underlies the ridge of skin immediately anterior to the clitoral hood (prepuce). The glans clitoris is positioned at the end of the body of the clitoris.  IDENTIFICATION OF STRUCTURES IN THE UROGENITAL TRIANGLE OF MEN In men, the urogenital triangle contains the root of the penis. The testes and associated structures, although they migrate into the scrotum from the abdomen, are generally evaluated with the penis during a physical examination. The scrotum in men is homologous to the labia majora in women. Each oval testis is readily palpable through the skin of the scrotum (Fig. 5.85A). Posterolateral to the testis is an elongated mass of tissue, often visible as a raised ridge that contains lymphatics and blood vessels of the testis, and the epididymis and ductus deferens. A midline raphe (Fig. 5.85B) is visible on the skin separating left and right sides of the scrotum. In some individuals, this raphe is prominent and extends from the anal aperture, over the scrotum and along the ventral surface of the body of the penis, to the frenulum of the glans.  The root of the penis is formed by the attached parts of the corpus spongiosum and the corpora cavernosa. The corpus spongiosum is attached to the perineal membrane and can be easily palpated as a large mass anterior to the perineal body. This mass, which is covered by the bulbo- spongiosus muscles, is the bulb of penis. The corpus spongiosum detaches from the perineal membrane anteriorly, becomes the ventral part of the body of the penis (shaft of penis), and eventually terminates as the expanded glans penis (Fig. 5.85C,D).  The crura of the penis, one crus on each side, are the attached parts of the corpora cavernosa and are anchored to the ischiopubic rami (Fig. 5.85E). The corpora cavernosa are unattached anteriorly and become the paired erectile masses that form the dorsal part of the body of the penis. The glans penis caps the anterior ends of the corpora cavernosa.