This is the system whereby life is reproduced by the fertilisation of a female egg, by male sperm. Following this, we have the subsequent implantation of the egg into the uterine wall, its nourishment, its development as a foetus, then its birth as a young animal.


The system consists of two testes enclosed in a scrotum, accessory organs and a penis. These are shown in the diagram below:

Figure 1: The Male Reproductive Tract

Source: ansci.wisc


The testes are contained in a multi-layered sac called the scrotum. This provides for a suitable environment for the production and maturation of the spermatozoa, or sperm cells.

The testes themselves consist of a mass of seminiferous tubules and connective tissue partitions, encased in a thick fibrous coat. Cells of Leydig are spread between the tubules. These produce the hormone named Testosterone which is also known as the “male hormone”.

The  seminiferous  tubules  unite  to  leave  the  testes  as  a  long,  highly  coiled  tube  known  as the epididymis. This is divided into the head, the body and the tail by which is shown in the diagram below.

The tail of the epididymis links up with the vas deferens. These muscular tubes pass from the testes into the abdominal cavity, together with the blood, lymphatic and nerve supply of the testes. This collection of vessels is known as the spermatic cord. If castration of the animal is required, this cord is severed or constricted.

Figure 2: The Testicle

The two vas deferens vessels enter into the urethra just after it passes from the bladder.


These consist of the following glands:

  • Ampullae glands
    • Seminal vesicles
    • Prostate gland and
    • Bulbs – urethral or Cowper’s Glands

All these glands contribute to the major part of the final ejaculation of the semen.


These are glandular enlargements of the two vas deferens vessels just before they enter the urethra.

Seminal Vesicles

These are paired glands emptying into the urethra together with the two vas deferens vessels.

Prostate Gland

This is a single gland virtually surrounding the urethra and emptying into it.

Cowper’s Glands

These are small paired glands on either side of the urethra. These Accessory Organs can be seen in the diagram below.

The Penis

This is the male organ of copulation and it is divided into 3 sections:

  • The glans or tip
    • The body or main portion
    • The roots (these two roots attach the penis to the pelvic bone).

The penis is covered in a fibrous capsule and consists of the urethra surrounded by cavernous or erectile tissue. This tissue comprises blood sinusorials separated by sheets of connective tissue.

Figure 3: The Structure of a Penis

In the stallion, the penis is relatively simple, consisting of a large amount of cavernous tissue in relation to connective tissue, so the penis becomes much larger in all directions upon erection.

Animals with a sigmoid or S-shaped flexure of the penis, e.g. the bull, the ram and the boar, have a higher proportion of connective tissue to erectile tissue. This results in a heavy capsule which distends very little on erection. The chief action of erection of the penis in these animals consists of lengthening of the penis by the straightening of the sigmoid flexure.


This is basically an increase in the turgidity of the penis caused by a greater in-flow of blood than out- flow. The arteries undergo vasco-dilation and the venous drainage vasco-constriction. Dilation of the arteries is achieved by stimulation of the nerves from the pelvic plexus. The decrease in venous drainage is caused partly by compression of the dorsal veins of the penis between the ischial arch and the body of the penis when the ischio-cavernous muscles contract.

On erection, the penis of the horse shows considerable increase in diameter as well as length because of the relatively large amount of erectile tissue in comparison with connective tissue. In those large animals with sigmoid flexure, erection is chiefly the straightening out of these flexures. There is relatively little erectile tissue, and so the length and diameter of the penis remains virtually the same.

The withdrawal of the penis after erection is achieved by the muscles.


This is the reflex emptying of the epididymis, urethra and accessory sex glands. This reflex is most commonly caused by the stimulation of the glans penis.


This term refers to the production and maturation of the spermatozoa or sperm cells. Immature sperm cells are produced by sex cells in the seminiferous tubules. These immature cells enter the epididymis. During their passage through this long coiled tube, which normally takes from 9 to 14 days, they divide and mature to become mobile spermatozoa capable of fertilizing ova. Spermatozoa are stored in the tail of the epididymis until emptied during ejaculation. Sperm cells become mobile only when exposed to oxygen during ejaculation. Mobility is achieved by the movement of the tail.

Figure 4: A mature Spermatozoon is shown below.


The chief hormone produced by the cells of Leydig in the testes is the male hormone, or testosterone.

Testosterone has the following functions:

  • Promotes development and function of the accessory organs
    • Causes development of secondary sex characteristics
    • It is necessary for mating activity, including the sex drive, penile erection and ejaculation
    • It is necessary for the maturation of sperm cells

These secrete seminal fluid which:

  • Provides transport of sperm cells
  • Provides a favourable medium for nutrition of the cells
  • Acts as a buffer against the acidity of the female tract