3o8 CHORDATE ANATOMY 



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 u;-ination, 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. 



B, I. Reproductive Organs: Female 



Ovaries. The ovaries are paired whitish almond-shaped organs from 

 one to two inches in length lying on the sides of the pelvis, usually with 

 the long axis parallel to that of the body. Each ovary is attached by a 

 thick membranous mesovarium to the broad ligament which supports 

 the uterus. The ovary is also attached to the side of the uterus by an 

 ovarian ligament. Another suspensory ligament carrying blood-vessels 

 and nerves extends upwards from the ovary along the wall of the pelvis. 



The ovary is covered by a layer of columnar epithelium, which is 

 apparently a special modification of the peritoneum. Most of the 

 substance of the ovary is a connective-tissue stroma containing some 

 smooth muscle fibers. In the cortex are ova in various stages of develop- 

 ment, surrounded by folHcle cells. As an ovum approaches maturity, 

 the follicular layer of cells which surrounds it increases in thickness. 

 Eventually, a liquid-filled cavity appears in the foUicle and the ovum is 

 crowded to one side. As the follicular liquid increases, the follicle migrates 

 toward the surface of the ovary. The pressure is finally sufficient to 

 burst the folUcle, and the ovum escapes into the uterine tube, the coelomic 

 opening of which, the ostium tubae, almost completely surrounds the 

 ovary. Sections of mature ovaries show Graafian follicles in various 

 stages of growth. That portion of the follicle which remains in the 

 ovary is converted into an endocrinal tissue, the corpus luteum. If 

 the ovum is not fertilized and therefore does not become implanted in the 

 uterus, the corpus luteum soon degenerates to form a corpus albicans, 

 which eventually disappears. (Fig. 274) Ovarian arteries and veins 

 enter the ovary besides a branch of the uterine artery. The nervous 

 supply is sympathetic. In addition to the reproductive function, the 

 ovaries serve as endocrinal organs. In ontogenesis the ovaries of the 

 embyro determine the secondary sexual characters of the female. 



Uterine Tubes. The uterine tubes, which convey the ova to the 

 uterus, vary in length from three to five inches. Near the uterus each 

 tube is straight, but as it approaches the ovary it becomes sinuous and 

 enlarged in diameter. The termination of each tube is a funnel-shaped 

 structure, the infundibulum, which opens into the body-cavity and 



