The =tunica vaginalis= is a somewhat pyriform serous sac which extends
from the internal inguinal ring through the inguinal canal to the bottom
of the scrotum. Like the abdominal peritoneum, of which it is an
evagination, it consists of two layers—parietal and visceral. The
=parietal layer=, or =tunica vaginalis communis= (s. reflexa), lines the
scrotum below; its narrow, tubular part lies in the inguinal canal and
is directly continuous with the parietal peritoneum of the abdomen at
the internal inguinal ring. The =cavity= of the tunica vaginalis (Cavum
vaginale) is a diverticulum of the general peritoneal cavity, with which
it communicates through the =vaginal ring= (Annulus vaginalis). It
contains normally a small quantity of serous fluid. The parietal layer
is reflected from the posterior wall of the inguinal canal around the
structures of the cord, forming the =mesorchium=, a fold analogous to
the mesentery of the intestine. The =visceral layer=, or =tunica
vaginalis propria=, covers the spermatic cord, testicle, and epididymis.
The =external cremaster muscle= (M. cremaster externus) lies on the
outer and posterior part of the tunic, to the scrotal part of which it
is attached.
Confusion has arisen from the use of the term internal abdominal or
inguinal ring in two senses. The term is used to designate the upper
or abdominal opening of the inguinal canal, but it is also applied to
the upper opening of the cavity of the tunica vaginalis. It should
only be applied to the abdominal opening of the canal, the
=subperitoneal ring=. The =peritoneal ring= at which the cavity of the
tunica vaginalis opens into the general peritoneal sac is
distinguished by the name =vaginal ring=. It is placed about four or
five inches (ca. 10 to 12 cm.) from the linea alba, and about two or
three inches (ca. 6 to 8 cm.) in front of the ilio-pectineal eminence.
In stallions it will usually admit the end of the finger readily, but
it may be abnormally large and allow a loop of bowel to enter the
cavity of the tunica vaginalis. In the gelding it is smaller and
sometimes partially occluded. In man the cavity is almost always
obliterated early, except in its scrotal portion, thus abolishing the
vaginal ring.
[Illustration:
FIG. 393.—DIAGRAM OF CROSS-SECTION OF SPERMATIC CORD AND TUNICA
VAGINALIS; LATTER REPRESENTED AS DISTENDED.
]