158 BIOLOGY OF THE LABORATORY MOUSE 



vascular fibrous connective tissue. The mucous membrane which is devoid 

 of glands forms longitudinal folds. The thin muscular coat contains some 

 inner circular and outer longitudinal smooth muscle fibers, which are inter- 

 mingled with considerable amounts of connective tissue. The wall is 

 covered by loose connective tissue. 



The vagina opens at the vulva. Immediately cephalad to the vaginal 

 orifice the clitoris forms a small elevation. The subcutaneous tissue of the 

 clitoris is rich in blood vessels, but does not contain any erectile tissue. 

 The clitoris contains a small pouch, the clitoral fossa, which is lined by 

 cornilied stratified squamous epithelium. The urethra opens on the dorsal 

 wall of this pouch, while ventro-laterally on each side open the ducts of the 

 two clitoral glands (Fig. 75). These glands are considerably smaller, but 

 similar in structure and position to the preputial glands of the male. The 

 excretory ducts are lined by stratified squamous epithelium. The sac-like 

 secretory alveoli are surrounded by a thin, connective tissue capsule, and 

 consist of large pale staining, often vacuolated cells which like all sebaceous 

 glands produce an oily secretion by cell degeneration. Each gland contains 

 a single hair folhcle with the hair shaft projecting into the duct (Fig. 81). 



Mammary Glands 



The mammary gland is a compound tubulo-alveolar gland. Mice have 

 five glands on each side, three in the thoracic and two in the abdomino- 

 inguinal region (Fig. 89). The gland undergoes several progressive and 

 regressive changes during the lifetime of a breeding female. In the male a 

 very small rudimentary duct system is present. 



In the formation of the nipples all three layers of the epidermis take part 

 (germinativum, granulosum and corneum). The skin covering the nipples 

 is thickened and forms circular folds which allow for the stretching of the 

 nipples at the time of nursing. In the formation of the duct system the 

 stratum germinativum takes part. One main duct leads from each nipple 

 into the subcutaneous fat pads and forms the collateral and terminal 

 branches. The fat pads form a framework and seem to limit the growth of 

 the fully developed glands. Each nipple with its main duct, collateral and 

 terminal branches is a separate unit and is not in communication with 

 the others. 



The ducts are lined by cuboidal epithelial cells which have dark staining, 

 oval nuclei and small amounts of cytoplasm. They are surrounded by 

 circularly arranged connective tissue fibers. The connective tissue coat is 



