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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
iliac arteries. The common iliac is represented by the upper third of this line, the external iliac by the remaining two-thirds.
  Of the larger branches of the abdominal aorta, the celiac artery is 4 cm., the superior mesenteric 2 cm. above the transpyloric line; the renal arteries are 2 cm. below the same line. The inferior mesenteric artery is 4 cm. above the bifurcation of the abdominal aorta.

Nerves.—The thoracic nerves on the anterior abdominal wall are represented by lines continuing those of the bony ribs. The termination of the seventh nerve is at the level of the xiphoid process, the tenth reaches the vicinity of the umbilicus, the twelfth ends about midway between the umbilicus and the upper border of the symphysis pubis. The first lumbar is parallel to the thoracic nerves; its iliohypogastric branch becomes cutaneous above the subcutaneous inguinal ring; its ilioinguinal branch at the ring.
 
9. Surface Anatomy of the Perineum
 

Skin.—In the middle line of the posterior part of the perineum and about 4 cm. in front of the tip of the coccyx is the anal orifice. The junction of the mucous membrane of the anal canal with the skin of the perineum is marked by a white line which indicates also the line of contact of the external and internal Sphincters. In the anterior part of the perineum the external genital organs are situated. The skin covering the scrotum is rough and corrugated, but over the penis it is smooth; extending forward from the anus on to the scrotum and penis is a median ridge which indicates the scrotal raphé. In the female are seen the skin reduplications forming the labia majora and minora laterally, the frenulum of the labia behind, and the prepuce of the clitoris in front; still more anteriorly is the mons pubis.

Bones.—In the antero-lateral boundaries of the perineum, the whole outline of the pubic arch can be readily traced ending in the ischial tuberosities. Behind in the middle line is the tip of the coccyx.

Muscles and Ligaments.—The margin of the Glutæus maximus forms the postero-lateral boundary, and in thin subjects, by pressing deeply, the sacrotuberous ligament can be felt through the muscle. The only other muscles influencing surface form are the Ischiocavernosus covering the crus penis, which lies on the side of the pubic arch, and the Sphincter ani externus, which, in action, closes the anal orifice and causes a puckering of the skin around it.
 
10. Surface Markings of the Perineum
 
  A line drawn transversely across in front of the ischial tuberosities divides the perineum into a posterior or rectal, and an anterior or urogenital, triangle. This line passes through the central point of the perineum, which is situated about 2.5 cm. in front of the center of the anal aperture or, in the male, midway between the anus and the reflection of the skin on to the scrotum.

Rectum and Anal Canal.—A finger inserted through the anal orifice is grasped by the Sphincter ani externus, passes into the region of the Sphincter ani internus, and higher up encounters the resistance of the Puborectalis; beyond this it may reach the lowest of the transverse rectal folds. In front, the urethral bulb and membranous part of the urethra are first identified, and then about 4 cm. above the anal orifice the prostate is felt; beyond this the vesiculæ seminales, if enlarged, and the fundus of the bladder, when distended, can be recognized. On either side is the ischiorectal fossa. Behind are the anococcygeal body, the pelvic surfaces of the coccyx and lower end of the sacrum, and the sacrospinous ligaments (Fig. 1228).

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