Anatomy of the stud dog
The testicles in the stud dog are housed in the scrotum, a skin sac located between the thighs. The dartos, a smooth muscle
and fibrous tissue layer, acts as a cutaneous muscle and also forms the septum. The scrotum is important as it houses the
testicles outside of the body, and enables the dog to regulate the temperature of the testicle through contraction of the
dartos. Contraction of the dartos causes the skin of the scrotum to wrinkle.
The testicles themselves are ellipsoid and lie within the scrotal cavity with the long axis directed craniocaudally. They
are tilted so that the cranial end is most ventral. Several layers of fascia, the most important being the tunica albuginea,
surround the testicles. This is a thick fibrous capsule. The function of the testicle is to produce spermatozoa. This is accomplished
by the seminiferous tubules. The tubules empty into a network of ductules, called rete testes and are eventually transported
to the epididymis. In order to produce spermatozoa, the testicles must be at a temperature lower than the normal body temperature
of the dog as mentioned before. The testicle is formed embryologically inside the body and migrates through an opening in
the inguinal region of the abdominal wall. If this does not occur, the testicle does not produce spermatozoa and the dog is
termed a cryptorchid.
Once the spermatozoa have been produced within the testicle, they move to the epididymis. The epididymis is basically a long
tube that stores and transports the maturing spermatozoa. It is divided into a head, body, and a tail. The head of the epididymis
is firmly attached at the cranial end of the testis. The body lies on the dorsolateral surface, while the tail is attached
to the caudal end of the testis. It is important to know the orientation of the epididymis for the diagnosis of testicular
torsion and other reproductive problems.
The tail of the epididymis gradually transforms into the ductus deferens. The ductus deferens ascends as a component of the
spermatic cord. Other structures housed within the spermatic cord include the testicular artery, the sympathetic nerve plexus,
and lymphatics. The visceral vaginal tunic surrounds these structures. Together the testicular artery and veins constitute
a counter current exchange system for simultaneously conserving body heat and keeping the testicles cool. The spermatic cord
enters the abdomen through the inguinal canal. The vaginal process, internal spermatic fascia, cremaster muscle, external
pudendal vessels, and the genitofemoral nerve pass from inside to outside through this opening as well. The cremaster muscle
is especially important as it provides yet another method of temperature regulation for the testicles, as well as a means
of protection. The cremaster, when contracted, pulls the testes and other contents of the fibrous tunic closer to the body
where they are less pendulous and vulnerable during physical activity. The inguinal canal itself consists of the deep inguinal
ring, the inguinal ligament, and the superficial inguinal ring.
The dog has only one accessory sex gland, the prostate. A median septum divides the gland into right and left lobes, which
are then further divided by septae into lobules. The prostatic capsule and septae contain smooth muscle fibers that contract
to expel prostatic fluid during ejaculation. The prostate is located within the pelvic canal. It surrounds the proximal end
of the urethra, caudal to the neck of the bladder and positioned between the rectum and symphysis pubis. Disseminated prostate
gland is present surrounding the pelvic urethra. The pelvic urethra itself lacks smooth muscle, but is rich in elastic tissue.
The penis of the dog is composed of numerous venous sinuses enclosed within a fibro elastic capsule termed the tunica albuginea.
Engorgement of the venous sinuses with blood increases internal pressure, which stretches the fibro elastic wall, rendering
it and the penis turgid. The caudal portion of the glans penis is called the bulb glandis. This portion of the penis continues
to swell following copulation, and its slow collapse after ejaculation, is responsible for the prolonged "tie" common between
mating dogs. The penis contains a bone surrounding the penile urethra. This is termed the os penis. The paired retractor penis
muscles run superficially along the caudal ventral surface of the penis and insert on the distal end of the body of the penis.
The penis is housed within the prepuce. The prepuce has an outer layer of skin and forms the preputial cavity. During erection,
the penis protrudes through the preputial orifice. The internal lamina pulls away from the preputial wall and coats the caudal
end of the free penis like skin. Fascicles of cutaneous trunci, called preputial muscle, leave the parent muscle and insert
on the lateral and ventral wall of the prepuce to assist return of the penis into the prepuce.