During early fœtal life the testicle is situated against the dorsal wall
of the abdominal cavity, in contact with the ventral surface of the
corresponding kidney. As growth proceeds it gradually migrates from this
primitive position, and finally passes down the inguinal canal into the
scrotum. Previous to its descent through the abdominal wall the testicle
is suspended by a fold of peritoneum, termed the =mesorchium=. This fold
contains the vessels and nerves of the testicle in its anterior border.
In its posterior edge is the elongated tail of the epididymis, and two
cords of fibrous tissue and unstriped muscle. One of these cords is
short and connects the tail of the epididymis with the testicle; later
it becomes shorter, and is termed the ligament of the epididymis. The
other cord, the =gubernaculum testis=, extends from the tail of the
epididymis to the subperitoneal tissue in the vicinity of the future
vaginal ring. The =deferential fold= (Plica ductus deferentis) is given
off from the inner face of the mesorchium, and joins the urogenital fold
posteriorly. The body of the epididymis at this time lies in the edge of
an oblique fold formed by the outer layer of the mesorchium. After the
middle of fœtal life, a pouch or diverticulum of the peritoneum, the
=processus vaginalis=, grows downward through the inguinal canal,
carrying with it cremaster fibers derived from the internal oblique
muscle and a layer from the transversalis fascia. It is accompanied by
an inguinal extension of the gubernaculum testis. The latter blends
below with the subcutaneous tissue which later becomes the dartos. The
tail of the epididymis first enters the processus vaginalis, followed by
the testicle with its mesorchium, which descends within this
diverticulum of the peritoneum until it reaches the scrotum. The vas
deferens and its fold descend synchronously with the epididymis and
testicle.
The mechanical factors concerned in the migration of the testicle are
matters on which much uncertainty still exists. That the gubernaculum
exerts sufficient traction to guide the epididymis and testicle to the
inguinal canal seems plausible. The internal inguinal ring may
constitute a locus minoris resistentiæ in the abdominal wall,
especially after the descent of the processus vaginalis. Progressive
shortening of the gubernaculum was formerly considered to be the chief
cause of the descent through the abdominal wall. Increase in the
intra-abdominal pressure is probably an important factor.
In the foal the descent of the testicles is often complete at birth, but
it frequently happens that one testicle or both may be retained in the
inguinal canal or in the abdomen for some months. In other cases the
testicle may return into the canal or abdomen, since in the young foal
the vaginal ring is large and the testicle small and soft, and not yet
closely anchored by the scrotal ligament. In rare cases the descent may
be completed as late as the fourth year (personal observation).
Indefinite retention of one testicle or both in the abdominal cavity
or inguinal canal is not rare in horses; this condition is termed
cryptorchism. Abdominal retention is the more usual form of
cryptorchism in adult horses, inguinal retention being usually
temporary. The retained testicle is usually, but not always, small,
thin, soft, and flabby, and is non-spermiogenic. The processus
vaginalis and the inguinal part of the gubernaculum are usually
present, but may be rudimentary. The ligament of the epididymis and
the corresponding part of the mesorechium are often so long that the
tail of the epididymis may be several inches distant from the
testicle. The abdominal part of the gubernaculum may be 8 to 10 inches
(ca. 20 to 25 cm.) long, and the ligament of the epididymis much
elongated (10 to 15 cm. in length according to Vennerholm); thus the
testicle may have a wide range. The vaginal ring is sometimes closed.
In many mammals the testicles normally remain in the abdominal cavity;
such animals are termed testiconda, and include the elephant, some
insectivora, hyrax, sloths, ant-eaters, armadillos, and cetacea. In
others the testicles descend periodically during the period of œstrum,
and then return into the abdomen, or they may be extruded and
retracted voluntarily; this is true of most rodents, many insectivora
(moles, shrews, hedgehog), and bats.