THE UROGENITAL SYSTEM 419 



Ureters. The renal ducts or ureters convey the secretions of the 

 kidneys to the bladder. Each ureter is a tube about a quarter of an 

 inch in diameter and about twelve inches long. Within the sinus of 

 the kidney, each ureter enlarges into a renal pelvis, which fills the sinus 

 and branches into the renal calyces. The human kidney has two major 

 calyces, superior and inferior, and each of these subdivides again in three 

 to five minor calyces. Each of the minor calyces is connected with one 

 or more renal papillae and in this way receives the urine from the col- 

 lecting tubules. The two ureters open into the bladder on its posterior 

 surface, about an inch from the beginning of the urethra. Three layers 

 appear in a cross section of an ureter, an inner mucosa of transitional 

 epithelium, a muscularis coat of longitudinal and circular muscles, and an 

 outer adventitia of loose connective tissue. 



The Bladder. The bladder is a muscular sac, the shape and size of 

 which vary greatly since, when distended with urine, it may increase 

 twenty to thirty times its size when empty. A median connective tissue 

 ligament, the urachus, a rudiment of the embryonic allantoic stalk, 

 extends from the bladder to the umbilicus. The hypogastric arteries 

 of the embryo degenerate to form lateral imibilical ligaments connected 

 with the bladder. Other ligaments in addition to these serve to hold 

 the bladder in position. Viewed from within, the dorsal wall of the 

 bladder shows a triradiate figure called the trigone, formed by three 

 ridges extending from the two openings of the ureters and from the 

 urethra. 



The lining of the bladder is a layer of transitional epithelium capable 

 of greater distension. Next to this is a layer of loose connective tissue 

 with many elastic fibers. The greater portion of the thickness of the 

 wall of the bladder is, however, a coat of smooth muscle fibers arranged 

 in three layers, to which nerve fibers are distributed from both the spinal 

 cord and the sympathetic system. At the origin of the urethra, a thick- 

 ened ring of muscle forms a sphincter. In the act of urination, nerves 

 stimulate the muscles of the bladder to contract, and inhibit the contrac- 

 tion of the sphincter muscle of the urethra. 



Urethra. The passage from the bladder to the external orifice is 

 the urethra. Its length differs in the two sexes, being about four inches 

 long in the male and an inch to an inch and a half in the female. In the 

 male urethra, three portions are distinguished, a prostatic portion, which 

 is surrounded by the mass of the prostate gland and into which the 

 ejaculatory ductus deferentes open; a membranous portion, which is 

 constricted where the urethra passes through the urogenital trigone or 

 diaphragm; and a cavernous portion surrounded by the corpus cavernosum 

 urethrae. The diameter of the urethra differs considerably among these 

 regions. 



