Page 540 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.2 The male reproductive tr act 515

VetBooks.ir are not externally palpable in stallions, all portions internal inguinal ring. Each cord contains the corre-
sponding deferent duct, testicular artery, testicular
of the epididymis are usually palpable through the
scrotal wall. The head of the epididymis may be dif-
ficult to ascertain because of its flattened nature and veins, lymphatic vessels and nerves. The cremaster
muscle is situated in the caudolateral borders of each
because the cremaster muscle lies on top of it. spermatic cord.
The histological structure of the epididymis The testicular artery descends through the ingui-
changes as it continues through its different regions, nal ring into the cranial border of the spermatic cord
with epithelial height being greatest proximally in a tortuous manner and divides near the testis into
and smooth muscle components greatest distally. several branches to supply the testis and epididymis.
As sperm are transported from the excurrent ducts These small branches, embedded in the tunica albu-
into the head, along the body and into the tail, they ginea, enter the parenchyma via the trabeculae and
undergo a number of morphological and physiologi- septa of the testis. A corresponding network of veins
cal changes that ultimately render them motile and leaves the testis and surrounds the testicular artery
fertile. Specific maturational changes include: the in a tortuous manner, forming the pampiniform
capacity for progressive motility; shedding of the plexus. This countercurrent arrangement of artery
cytoplasmic droplet; plasma and acrosomal mem- and veins, as in other species, is responsible for much
brane alterations; DNA stabilisation; and metabolic of the thermoregulation of the testis in the stallion.
changes. The cooler venous blood surrounding the testicular
The tail of the epididymis generally serves to artery transfers heat away from the testicular arte-
store the matured sperm (Fig. 2.123). rial blood to the venous side, much like a counter-
current heat exchanger. Blood temperature within
SPERMATIC CORD AND VASCULAR the branches of the testicular vein is lowered to
SUPPLY TO THE TESTIS (FIG. 2.124) less than core body temperature by evaporative heat
loss through the skin. As a result, blood within the
Each spermatic cord is enveloped in the parietal layer testicular artery is several degrees cooler on reach-
of the vaginal tunic, which extends distally from the ing the testicle.

2.124
Pampiniform plexus

Testicular nerves and lymphatic vessels
Visceral layer of vaginal tunic
Parietal layer of vaginal tunic
Cremaster muscle
Testicular artery
Spermatic fascia
Vaginal cavity
Mesorchium

Mesoductus
Ductus deferens

Fig. 2.124 Schematic
representation of a transverse
section of the spermatic cord.