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 lymph 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 em.), 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 pounds (ca. 4 kg.), according to Elle nberger. The broad ligaments increase greatly 
in size and contain more muscular tissue. The vessels are greatly enlarged and form new brane hes. 
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 em.) 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 dorsally to the rectum, ventrally to the bladder and urethra, and 
laterally to the pelvie wall. The recto-genital pouch of the peritoneum commonly 
extends between the vagina and rectum for a distance of about two inches (5 em.), 
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.t 
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 practically 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 vagina. 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 vagine). 
Vessels and Nerves.—The arteries are branches of the internal pudie arteries. 
The veins form a rich plexus which is drained by the internal pudic veins. The 
lymph vessels go to the internal iliac lymph glands. The nerves are derived from 
the sympathetic through the pelvic plexus; numerous ganglia are present in the 
adventitia. 
1The 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 em.); when they are full, the 
vagina may be completely retroperitoneal or nearly so. 
Tn formalin-hardened subjects there is frequently a pronounced ring-like constriction at the 
junction of the vulva and vagina. 
