The Vulva ' 33 



sists, either as perpendicular shreds or in a broad expanse closing 

 one-half or more of the genital canal. We have met an instance 

 in a filly where it prevented copulation until after surgical inter- 

 vention occurred, and cases are recorded of imperforate hymen 

 which caused an accumulation of utero-vaginal secretions. 



6. The Vulva. 



The vulva, located immediately beneath the anus, constitutes 

 the posterior termination of the genital canal and, instead of being 

 derived from the mesodermic Muellerian ducts, as in case of the 

 preceding organs, its epithelial covering originates from the epi- 

 derm of the embryo. It opens externally by means of a verti- 

 cally elongated .slit, bounded upon either side by the labise vulvae, 

 which meet above and below to form the superior and inferior 

 vulvar cohimissures. 



The vulvar labise are covered by a very fine skin, in which the 

 growth of hairs maj^ be inconspicuous, as in the mare, while in 

 others, like the cow, there is a prominent tuft of hairs about the 

 inferior commissure. Since the parts are so scantily haired, 

 their color, which, as a rule, is simpl}' that of the adjacent skin, 

 becomes very conspicuous. In mares with white faces, eyes and 

 feet, the cutaneous covering of the vulva tends also to be devoid 

 of pigment. 



The prominence of the pigmentation of the skin of this part 

 serves as an aid in the clinical diagnosis of the venereal diseases 

 of horses (which see), in which important discolorations occur. 



The muscles of the vulva are chiefly circular and are divided 

 into two groups, the posterior and anterior constrictors. The 

 posterior constrictor is situated within the vulvar lips and con- 

 stitutes a true sphincter, analogous to those about other body 

 openings. Above, its fibers become lost in the perineum and the 

 sphincter ani ; below some go to the base of the clitoris 

 and some pass downwards to become lost in the skin and 

 other tissues on the inside of the thighs. The office of this 

 group comprises the usual function of a sphincter, their contrac- 

 tion bringing about the clbsure of the vulvar opening. The an- 

 terior group of muscle fibers invests the vulva in the region of 

 the hymen, just anterior to the meatus urinarius, where, by their 



3 



