226 THE FEMALE REPRODUCTIVE SYSTEM 



to the lumen is composed of several layers of fiat degenerating cells 

 little, if at all, cornified; the middle zone has several layers of flat 

 cells with a more granular content, suggesting the stratum granu- 

 losum of the epidermis ; the deepest zone has a number of cell layers 

 comparable to the stratum germanitivum of the epidermis. The 

 basal layer of cells are short columnar and ])()lyhedral in shape and 

 are active in giving rise to new cells. Between adjacent cells of 

 the upper layers of the basal zone there may be intercellular bridges, 

 giving the appearance of prickle cells found in the epidermis. In the 

 tunica propria are many leukocytes, especially lymphocytes. Smears 

 from the vagina of some rodents show a series of changes, and the 

 type of cells cast into the lumen indicates the progress of ovulation. 

 Some authors recognize a submucosa below the tunica propria as 

 a connective tissue of looser texture between it and the muscularis. 

 The connective tissue of the adventitia connects the vagina with 

 surrounding structures. 



The Vestibule.— The vestibule is a shallow depression posterior 

 to the vagina. Its surface is covered with stratified ei)ithelium, 

 which rests on a tunica propria of fibroelastic connecti\e tissue. 

 Below the tunica propria are muscles of the skeletal variety. A 

 pair of vascular bodies form the clitoris adjacent to the entrance of 

 the vagina. This is a structure similar to the corpora cavernosa 

 of the penis and functions as erectile tissue in periods of sexual 

 excitation. Several small glands are also present in the connective 

 tissue around the opening to the vestibule, and their cohunnar cells 

 secrete a mucous substance. The Bartholin glands which occur 

 on either side of the vestibule may be homologous with the bulbo- 

 urethral of the male. 



CESTRUS. 



In describing the ovary we followed the de^'elopment of ova 

 from the primitive germ cells to the time of their liberation, noting 

 that such ovarian activity varied considerably in different forms. 

 Usually there is a regular cyclical appearance of ovulation; in some 

 forms it occurs annually, but in others it occurs at shorter interxals. 

 Among the lower aquatic vertebrates, as in the fish, the mature 

 eggs are discharged into the water during a spawning season, at 

 which time males and females congregate. (\)i)ulati()n d(M's not 

 occur, but sperm are discharged into the water in the vicinity of 

 the eggs so that fertilization and further de\eloi)ment are external. 



