514 GENITAL ORGANS OF THE MARE 



in size and contain more muscular tissue. The vessels are greatly enlarged and form new branches. 

 The muscular coat, in spite of the increase in size and number of the fibers, is somewhat thinner, 

 except in the neck. The mucous membrane is thicker and more vascular; the epithelium be- 

 comes non-ciliated and stratified. 



THE VAGINA 



The vagina is the passage which extends horizontally through the pelvic 

 cavity from the neck of the uterus to the vulva. It is tubular, is about six to eight 

 inches (ca. 15 to 20 cm.) in length, and, when slightly distended, about four or five 

 inches (ca. 10 to 12 cm.) in diameter. Its dilatability appears to be limited only by 

 the pelvic wall. There is no external line of demarcation between the vagina 

 and the uterus or the vulva. 



It is related dorsally to the rectum, ventrally to the bladder and urethra, and 

 laterally to the pelvic wall. The recto-genital i^ouch of the peritoneum extends 

 between the vagina and rectum for a distance of about two inches (5 cm.), and 

 ventrally the vesico-genital jiotich passes backward somewhat less between the 

 vagina and bladder. Thus most of the vagina is retroperitoneal and is sur- 

 rounded by a quantity of loose connective tissue, a venous plexus, and a variable 

 amoimt of fat. 



Structure. — With the exception of the short peritoneal part, as indicated above, 

 the proper wall of the vagina is composed of musctilar and mucous coats. The 

 muscular coat is composed of a thin layer of longitudinal fillers, and a thicker layer 

 of circular fibers; it contains also many elastic fibers. The mucous coat is highly 

 elastic, and is covered with a stratified epithelium. It has no glands. 



Under usual conditions the cavity is practically obliterated by apposition of 

 the walls, so that the Ivmien is a transverse slit; this condition is ]ironounced when 

 the ampulla of the rectum is full. The anterior end of the vagina is largely occupied 

 by the intravaginal part of the neck of the uterus, so that the cavity is here reduced 

 to an annular recess termed the fornix vaginae. The posterior part is directly 

 continuous with the vulva without any line of demarcation except the valvular 

 fold, which covers the external urethral orifice; in very young subjects this fold is 

 continued on either side, forming the hymen, which narrows the entrance to the 

 vagina (Introitus vaginae).^ 



Vessels and Nerves. — The l)lood-supply is derived from the branches of the 

 internal pudic arteries. The veins form a jjlexus which is drained by the internal 

 pudic veins. The lymphatics pass to the internal iliac lymph glands. The nerves 

 are derived from the sympathetic through the pelvic plexus. 



THE VULVA 



The vulva or urogenital sinus is the terminal part of the genital tract." It is 

 continuous in front with the vagina, and opens externally at the vulvar cleft 

 below the anus. There is no external line of demarcation between the vagina and 

 vulva. The tube is about five inches (ca. 12 cm.) in length, measured from the 

 external urethral orifice to the inferior commissure; dorsally it is considerably 

 shorter. It is related dorsally to the rectum and anus, ventrally to the pelvic floor, 

 and laterally to the sacro-sciatic ligament, the semimembranosus mtiscle, and the 

 internal pudic artery. The external orifice or vulvar cleft (Rima vulvae) has the 

 form of a vertical slit, about four to six inches (ca.lO to 1.5 cm.) high, and is margined 



1 In formalin-hardened subjects there is frequently a pronounced ring-like constriction at the 

 beginning of the vulva. 



- The term vulva is used here in the sense in which it is understood generally l>y English 

 and French veterinarians. In the German works it is applied only to the laliia and other struc- 

 tures around the external orifice of the urogenital sinus, while the sinus itself is termed the vestibule 

 of the vagina (Vestibulum vagina^). 



