The =testicles= (Testes) are situated in the inguinal region, inclosed
in a diverticulum of the abdomen termed the scrotum. Their long axes are
nearly longitudinal. They are ovoid in form, but considerably compressed
from side to side. Each presents two surfaces, two borders, and two
extremities. The =internal= and =external surfaces= (Facies medialis,
lateralis) are convex and smooth. The =free= or ventral =border= (Margo
liber) is convex in both directions. The =attached= or dorsal =border=
(Margo epididymidis) is nearly straight, and is the one by which the
gland is suspended in the scrotum by the spermatic cord; the epididymis
is attached to this border and overlies it externally. The =anterior=
and =posterior extremities= (Extremitas capitata, caudalis) are rounded.
At the anterior extremity there is often a sessile or pedunculated sac
which contains a clear fluid; this is the =appendix testis= or hydatid
of Morgagni, from which a thread-like process extends backward toward
the vas deferens. It is regarded as a remnant of the Müllerian duct of
the embryo.
A testicle of average size of an adult stallion is about four or five
inches (ca. 10 to 12 cm.) long, two and a half inches (ca. 6 to 7 cm.)
high, and one and a half to two inches (ca. 4 to 5 cm.) wide; it weighs
about ten and one-half ounces (ca. 300 grams). They vary much in size in
different subjects, and are commonly of unequal size, the left one being
more often the larger.
The =epididymis= is adherent to the attached border of the testicle, and
overlaps somewhat the external surface. Its anterior enlarged end is
termed the =head= or globus major (Caput epididymidis), and its
posterior, slightly enlarged end is the tail or globus minor (Cauda
epididymidis); while the intermediate narrow part is the =body= (Corpus
epididymidis). The head is closely connected with the testicle by the
efferent ducts of the latter, by connective tissue, and by the serous
membrane. The body is less closely attached by the serous covering,
which forms externally a pocket beneath the epididymis termed the
=digital fossa= (Sinus epididymidis). The tail is firmly attached to the
posterior extremity of the testicle by a short ligament (Lig.
epididymidis), formed by a thick fold of the tunica vaginalis, which
contains smooth muscle-fibers; it is continued by the vas deferens.
=Structure of the Testicle and Epididymis.= The greater part of the
surface of the testicle is covered by a serous membrane, the =tunica
vaginalis propria=, which is the visceral layer of the serous envelop of
the cord and testicle; this is reflected from the attached border of the
gland, leaving an uncovered area at which the vessels and nerves in the
spermatic cord reach the testicle. Beneath this serous covering is the
=tunica albuginea=, a strong capsule composed of dense white inelastic
fibrous tissue and unstriped muscle-fibers. From the deep surface of
this tunic delicate membranous =septa= (Septula testis) pass into the
gland, dividing it into pyramidal spaces, the bases of which are
peripheral. At the anterior part of the attached border of the testicle
connective tissue and smooth muscle-fibers pass into the substance of
the gland, forming the =mediastinum testis=; from this the septa
radiate. The mediastinum contains a network of fine canals, the =rete
testis=; it is traversed by blood-vessels, which radiate on the septa to
the deep face of the tunica albuginea, forming a delicate network—the
so-called tunica vasculosa. The spaces imperfectly marked off by the
septa contain the =parenchyma testis=, which consists of =seminiferous
tubules= (Tubuli seminiferi), supported by loose intertubular connective
tissue. The tubules are collected into small masses or =lobules= (Lobuli
testis). They are at first very tortuous (Tubuli contorti); toward the
apex of the lobule they unite with other tubules, forming larger
straight tubules (Tubuli recti); these pass into the mediastinum testis,
where they form the =rete testis=. From the latter a number (ca. 15 to
20) of larger =efferent tubules= (Vasa efferentia) pierce the albuginea
and enter the head of the epididymis.
[Illustration: