602 GENITAL ORGANS OF THE MARE 



there is also a branch from the internal pudic artery. The veins form pampini- 

 form plexuses and accompany the arteries. The Ijrmph vessels are numerous and 

 go to the internal iliac and lumbar glands. The nerves are derived from the 

 sympathetic through the uterine and pelvic plexuses. 



The foregoing statements refer to the non-gravid uterus. In the pregnant state it under- 

 goes important changes in size, position, and structure. The increase in size affects chiefly the 

 gravid horn (except in the case of twins) and the body. The horn attains a length of about 

 two and a half to three feet (ca. 80 to 90 cm.), and a corresponding diameter; in this process it 

 extends much beyond the ovary and the broad ligament. The gravid uterus is entirely abdominal 

 in position, and extends along the ventral wall, chiefly to the left of the median plane. It weighs 

 about nine poxmds (ca. 4 kg.), according to EUenberger. The broad ligaments increase greatly 

 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 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. 15-20 cm.) in length, and, when slightly distended, about four or five 

 inches (ca. 10-12 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 dorsallj^ to the rectum, ventrally to the bladder and urethra, and 

 laterally to the pelvic wall. The recto-genital pouch of the peritoneum commonly 

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

 and ventrally the vesico-genital pouch passes backward somewhat further 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 

 amount of fat.^ 



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

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

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

 of circular fibers; it is covered externally by a fibrous adventitia, and there is a 

 large amount of intermuscular connective tissue. The mucous coat is highly 

 elastic, and is covered with a stratified — but not squamous — epithelium. It has 

 no glands. 



Under usual conditions the cavity is practicallj^ obliterated by apposition of 

 the walls, so that the lumen is a transverse slit ; this condition is pronounced when 

 the rectum is full. The anterior end of the vagina is largely occupied by the intra- 

 vaginal part of the neck of the uterus, so that the cavity is here reduced to an an- 

 nular recess termed the fornix vaginae. The posterior part is directly continuous 

 with the vulva without any line of demarcation except the transverse 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 (In- 

 troitus vaginffi) } 



Vessels and Nerves. — The arteries are branches of the internal pudic arteries. 

 The veins form a rich plexus which is drained by the internal pudic veins. The 

 lymph vessels go to the internal ihac lymph glands. The nerves are derived from 

 the sympathetic through the pelvic plexus; numerous ganglia are present in the 

 adventitia. 



'The amount of the vagina which is covered by peritoneum varies, depending apparently 

 on the degree of fullness of the rectum and bladder. When these organs are empty, the peritoneum 

 may cover the vagina for a distance of three or four inches (ca. 8-10 cm.); when they are full, the 

 vagina may be completely retroperitoneal or nearly so. 



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

 junction of the vulva and vagina. 



