The Vulva 4 1 



3. The Vulva 



The vulva, located immediately beneath the anus, consti- 

 tutes the posterior termination of the genital canal. In- 

 stead of being derived from the mesodermic Muellerian 

 ducts like the preceding organs, the vulvar epithelium origi- 

 nates from the ectoderm of the embryo. 



Anteriorly, the boundary between the vagina and the vulva 

 is marked by the hymen, a transverse membranous expanse 

 stretching across the genital canal, which represents the 

 partition between the termination of the hind gut and the 

 proctodeum of the embryo, in the lower, or genito-urinary, 

 division of the cloaca. Generally it atrophies and almost or 

 wholly disappears in domestic animals before birth, but at 

 times it persists, either as vertical bands stretching across 

 from the roof to the floor of the vagina or as a circular ex- 

 panse largely closing the genital canal except in its center. 

 Rarely in domestic animals the hymen persists as an im- 

 pervious membrane. 



The vulva opens externally by means of a vertically elong- 

 ated slit, bounded upon either side by the labiae vulvae, 

 which meet above and below to form the superior and in- 

 ferior vulvar commissures. 



The vulvar labiae are covered externally by a very fine 

 skin. In the mare the growth of hairs is inconspicuous ; in 

 the cow, there is a prominent tuft of hairs about the in- 

 ferior commissure. The parts are so scantily haired that 

 their color, which as a rule is simply that of the adjacent 

 skin, becomes very conspicuous. In dark-skinned mares 

 with white faces, eyes, and feet, the cutaneous covering of 

 the vulva tends also to be devoid of pigment. The promi- 

 nence of the pigmentation of the skin of this part serves as 

 an aid in the clinical diagnosis of the venereal diseases of 

 horses, in which important discolorations occur. 



The muscles of the vulva, which are chiefly circular, are 

 divided into two groups — the posterior and anterior con- 

 strictors. The posterior constrictor, situated within the 

 vulvar lips, constitutes a true sphincter, analogous to those 



