Gross Anatomy - Perineum and Pelvis summary

• The perineum is a diamond-shaped space that has the same boundaries as the pelvic outlet or the inferior aperture of the pelvis and is bounded by the pubic symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligament, and the tip of the coccyx.
• It is divided into urogenital and anal triangles.
• The superficial perineal space (pouch) lies between the inferior fascia of the urogenital diaphragm (perineal membrane) and the superficial perineal fascia (Colles' fascia) and contains perineal muscles, the crus of the penis or clitoris, the bulb of the penis or vestibule, the central tendon of the perineum, the greater vestibular glands in the female, branches of the internal pudendal vessels, and the pudendal nerve.
• The deep perineal space (pouch) lies between the superior and inferior fasciae of the urogenital diaphragm. It contains the deep transverse perineal muscle and sphincter urethrae, the membranous part of the urethra, the bulbourethral glands in the male, and branches of the internal pudendal vessels and pudendal nerve.
• The ischiorectal fossa is separated from the pelvis by the levator ani and its fasciae and is bounded by the sphincter urethrae and deep transverse perineal muscles (anteriorly), the gluteus maximus and the sacrotuberous ligament (posteriorly), the sphincter ani externus and levator ani (superomedially), the obturator fascia covering the obturator internus (laterally), and the skin (floor). It contains the inferior rectal nerve and vessels and fat.

• The scrotum is a sac of skin with no fat and the dartos muscle (fascia), which is continuous with the superficial penile fascia and superficial perineal fascia; contains the testis and epididymis and receives blood from the external and internal pudendal arteries; and is innervated by the anterior scrotal branch of the ilioinguinal nerve, the genital branch of the genitofemoral nerve, the posterior scrotal branch of the perineal branch of the pudendal nerve, and the perineal branch of the posterior femoral cutaneous nerve. Lymphatics in the scrotum drain into the superficial inguinal nodes. The dartos muscle, cremaster muscle, and pampiniform plexus help regulate the temperature of the testes in the scrotum; the dartos muscle is responsible for wrinkling the scrotal skin, whereas the cremaster muscles are responsible for elevating the testes. The scrotal skin wrinkles to increase its thickness and reduce heat loss.
• The penis consists of a root, which includes two crura and the bulb of the penis, and the body, which contains the single corpus spongiosum and the paired corpora cavernosa. Its head is called the glans penis , which is formed by the terminal part of the corpus spongiosum.

• The labia majora are two longitudinal folds of skin that are homologous to the scrotum and contain the terminations of the round ligaments of the uterus. The labia minora are hairless and contain no fat. They are divided into an upper (lateral) part , which fuses above the clitoris to form the prepuce of the clitoris, and a lower (medial) part , which fuses below the clitoris to form the frenulum of the clitoris. The vestibule of the vagina is the space between the labia minora and has the openings for the urethra, vagina, and ducts of the greater vestibular glands in its floor.
• The clitoris is homologous to the penis and consists of two crura, two corpora cavernosa, and a glans but no corpus spongiosum. The glans clitoris is derived from the corpora cavernosa and is covered by a sensitive epithelium.

• The pudendal nerve passes through the greater sciatic foramen between the piriformis and coccygeus muscles and enters the perineum with the internal pudendal vessels through the lesser sciatic foramen. The pudendal nerve enters the pudendal canal, gives rise to the inferior rectal and perineal nerves, and terminates as the dorsal nerve of the penis (or clitoris).
• The inferior rectal nerve innervates the sphincter ani externus and the skin around the anus.
• The perineal nerve divides into a deep branch, which supplies all of the perineal muscles, and a superficial (posterior scrotal or labial) branch, which supplies the scrotum or labia majora.
• The dorsal nerve of the penis or clitoris runs between the two layers of the suspensory ligament of the penis or clitoris and runs deep to the deep fascia on the dorsum of the penis or clitoris to innervate the skin, prepuce, and glans.
• The internal pudendal artery is accompanied by the pudendal nerve during its course, leaving the pelvis by way of the greater sciatic foramen and entering the perineum through the lesser sciatic foramen. It gives rise to the inferior rectal, perineal, and urethral arteries and the artery of the bulb, deep artery of the penis or clitoris, and dorsal artery of the penis or clitoris.
• The internal pudendal vein arises from the lower part of the prostatic venous plexus in the male or the vesical plexus in the female and usually empties into the internal iliac vein by a common trunk.
• The deep dorsal vein of the penis is an unpaired vein that begins behind the glans and lies in the dorsal midline deep to the deep fascia and superficial to the tunica albuginea, leaves the perineum through the gap between the arcuate pubic ligament and the transverse perineal ligament, and drains into the prostatic and pelvic venous plexuses. The superficial dorsal vein of the penis runs toward the pubic symphysis and terminates
  in the external (superficial) pudendal veins, which drain into the greater saphenous vein. The deep dorsal vein of the clitoris is small but also runs in the median plane between the left and right dorsal arteries and ends in the lower part of the vesical venous plexus.

• Basin-shaped ring of bone formed by the two hip bones, the sacrum and the coccyx. The hip or coxal bone consists of the ilium, ischium, and pubis. It is divided by the pelvic brim into the pelvis major (false pelvis) above and the pelvis minor (true pelvis) below.
  
• The pelvic diaphragm is formed by the levator ani and coccygeus, forms the pelvic floor, and supports all of the pelvic viscera. It flexes the anorectal canal during defecation, helps the voluntary control of micturition, and also helps direct the fetal head toward the birth canal at parturition.
• The broad ligament extends from the uterus to the lateral pelvic wall; serves to hold the uterus in position; and contains the uterine tube, uterine vessels, round ligament of the uterus, ovarian ligament, ureter, nerve plexus, and lymphatic vessels. It does not contain the ovary but gives attachment to the ovary through the mesovarium.
• The round ligament of the uterus is the remains of the lower part of the gubernaculum, runs within the broad ligament, and keeps the uterus anteverted and anteflexed. It enters the inguinal canal at the deep inguinal ring, emerges from the superficial inguinal ring, and becomes lost in the labium majus. The ovarian ligament extends from the ovary to the uterus below the uterine tube within the layers of the broad ligament.
• The lateral or transverse cervical (cardinal or Mackenrodt's) ligament of the uterus extends from the cervix and the vagina to the pelvic wall and contains smooth muscle fibers and supports the uterus.

• The ureter has three constrictions along its course: at the origin where the pelvis of the ureter joins the ureter, where it crosses the pelvic brim, and at its junction with the urinary bladder. It is accompanied in its course by the uterine artery , and thus, it is sometimes injured by a clamp during surgical procedures and may be ligated and sectioned by mistake during a hysterectomy. It can be remembered by the mnemonic, water (ureter) runs under the bridge (uterine artery). In the male, it passes posterior and inferior to the ductus deferens and lies in front of the seminal vesicle. Therefore, the ureter runs under the uterine artery in the female and the ductus deferens in the male. It courses obliquely through the bladder wall and functions as a check valve to prevent reflux of urine into the ureter.
• The urinary bladder: the uvula is a small rounded elevation just behind the urethral orifice at the apex of its trigone, and the trigone is bounded by the two orifices of the ureters and the internal urethral orifice, around which is a thick circular layer called the internal sphincter (sphincter vesicae). The musculature (bundles of smooth muscle fibers) is known as the detrusor muscle of the bladder. The bladder receives blood from the superior and inferior vesical arteries, and its venous blood is drained by the prostatic or vesical plexus of veins, which empties into the internal iliac vein; it is innervated by nerve fibers from the vesical and prostatic plexuses.
• Micturition (urination) is initiated by stimulating stretch receptors in the detrusor muscle in the bladder wall by the increasing volume of urine. Afferent (GVA) impulses arise from the stretch receptors in the bladder wall and enter the spinal cord via the pelvic splanchnic nerves. Sympathetic fibers induce relaxation of the bladder wall and constrict the internal sphincter, inhibiting emptying; parasympathetic fibers in the pelvic splanchnic nerve induce a contraction of the detrusor muscle and relaxation of the internal sphincter, enhancing the urge to void; somatic motor fibers in the pudendal nerve cause voluntary relaxation of the external urethral sphincter, and the bladder begins to void. At the end of micturition, the external urethral sphincter contracts, and bulbospongiosus muscles in the male expel the last few drops of urine from the urethra.

• The testis develops in the posterior wall of the embryo, descends into the scrotum retroperi-toneally and is covered by the tunica albuginea. The germ cells produce sperms; sustentacular (Sertoli) cells secrete androgen-binding protein and the hormone inhibin; interstitial (Leydig) cells secrete sex hormones; and myoid cells help to squeeze sperms through the tubules. The testis is supplied by the testicular artery from the abdominal aorta and is drained by veins of the pampiniform plexus into the inferior vena cava on the right and the renal vein on the left. Lymph vessels ascend with the testicular vessels and drain into the lumbar (aortic) nodes.
• The epididymis consists of a head, body, and tail and contains a convoluted duct. It functions in the maturation and storage of spermatozoa in the head and body and the propulsion of the spermatozoa into the ductus deferens.
• The ductus deferens enters the pelvis at the lateral side of the inferior epigastric artery; passes superior to the ureter near the wall of the bladder; is dilated to become the ampulla; joins the duct of the seminal vesicle to form the ejaculatory duct, which empties into the prostatic urethra on the seminal colliculus just lateral to the prostatic utricle; and transports and stores spermatozoa. During ejaculation, the thick layers of smooth muscle in the wall of the ductus deferens propel sperm into the urethra by peristalsis.
• The seminal vesicles are lobulated glandular structures that lie inferior and lateral to the ampullae of the ductus deferens and that contain (a) a sugar (fructose) and other nutrients that nourish the sperm, (b) prostaglandins that stimulate contraction of the uterus to help move sperm through the female reproductive tract, (c) substances that enhance sperm motility and suppress the immune response against semen in females, and (d) enzymes that clot the ejaculated semen in the vagina and then liquefy it so that the sperm can swim out. The seminal vesicles produce the alkaline constituent of the seminal fluid, which contains fructose and choline.
• The prostate gland is located at the base of the urinary bladder, and its secretion helps to clot and then to liquefy the semen. It has five lobes, including the anterior lobe, middle lobe (prone to benign hypertrophy), lateral lobes, and posterior lobe (prone to carcinomatous transformation).
• Erection and ejaculation are often described using a popular mnemonic device: point (erection by parasympathetic) and shoot (ejaculation by sympathetic).
  

• The ovaries are almond-shaped structures that lie on the lateral walls of the pelvic cavity, are suspended by suspensory and round ligaments, and produce oocytes or ova and steroid hormones.
• The uterine tube extends from the uterus to the ovary and consists of the isthmus, ampulla, and infundibulum. The fimbriated distal end creates currents, helping draw an ovulated oocyte into the uterine tube.
• The uterus contains a fundus, body, isthmus, and cervix and is supported by the broad, transverse cervical (cardinal), and round ligaments and the muscles of the pelvic floor, which provide the most important support. The uterine wall consists of the perimetrium, myometrium, and endometrium; the uterine cycle includes the menstrual, proliferative, and secretory phases; the first two phases are a shedding and then a rebuilding of endometrium in the 2 weeks before ovulation, and the third phase prepares the endometrium to receive an embryo in the 2 weeks after ovulation.
• The vagina extends between the vestibule and the cervix of the uterus, serves as the excretory channel for the products of menstruation, receives the penis and semen during coitus, and acts as the birth canal. The vaginal fornix is a ring-like recess around the tip of the cervix in the upper vagina.

• The rectum extends from the sigmoid colon to the anal canal; receives blood from the superior, middle, and inferior rectal arteries; and drains its venous blood into the portal venous system via the superior rectal vein and into the caval system via the middle and inferior rectal veins. The feces are stored in the ampulla, which is the lower dilated part of the rectum that lies above the pelvic diaphragm.
• The anal canal divides into an upper two thirds (visceral portion), which belongs to the intestine, and a lower one third (somatic portion), which belongs to the perineum. A point of demarcation between visceral and somatic portions is called the pectinate line, which is a serrated line following the anal valves. Hilton's white line is the intermuscular (intersphincteric) groove between the lower border of the internal anal sphincter and the subcutaneous part of the external anal sphincter.