How does the genital system of the stallion and the mare function? How is the urinary system of the horse constituted? An understanding of these different parts of the horse’s anatomy, even if only in general terms, is of interest, particularly in view of the many diseases that can affect the horse’s male and female reproductive systems or the animal’s nervous system.
The urinary system of the horse
The kidneys are simple and smooth. The left one, more caudal, is placed in the sub-lumbar region and the right one at the thoraco-lumbar junction. The ureters join the ceiling of the bladder which has a reduced capacity in relation to the size of the animal and only protrudes above the floor of the pelvis when full. The urethra extends backwards from the neck of the bladder. This canal, 2 or 3 cm in diameter, has a very different layout in males and females. In the mare it quickly joins the floor of the vestibule of the vagina. In the male, it is very long and is associated with the genitalia.
The male genitalia in the horse
The testicles, ovoid, side by side in a horizontal position, but slightly uneven, are slightly detached in the inguinal region. They are capped by the epididymis and the whole is protected by testicular envelopes which are distinguished into superficial, developing in situ, and deep, entrained during testicular migration, from the abdomen to the bottom of the scrotum. The superficial envelopes include the skin of the scrotum, which is supple, forming a bilobed sac common to both glands, and the dartos, which is attached to the scrotum, thick, resistant, with two independent sacs. These sacs are covered laterally by the cremaster muscle. They are lined internally by the external spermatic fascia.
The deep envelopes include a vaginal sheath, the dilated bottom of which contains the testicle and the neck of which, by elongating into a canal, opens into the abdomen through a vaginal ring, never obliterated. This sheath is made up of two layers: the visceral layer, which is very attached to the gland and its dependencies, and the parietal layer, which is directly continuous with the peritoneum. This sheet is lined externally by a thick layer of connective tissue, the internal spermatic fascia from which it cannot be separated, thus constituting the fibro-serosa.
The spermatic tract begins with the epididymis with an adherent head in front of the testis, a dorsal body, and a large, detached tail. From there the deferential duct, of reduced calibre, ascends into the abdomen and then the pelvis where it bulges into a voluminous deferential ampulla before reaching the ceiling of the urethra. The vesicular glands, ovoid above the bladder, are annexed to these ducts and are then stretched into ducts. These ducts, in association with the deferential ampullae, form short ejaculatory ducts which open side by side on an internal bulge at the origin of the urethra.
The urethra in the male is clearly divided into two portions: one intra-pelvic and one penile.
The intrapelvic portion is muscular, lined with a yellow mucous membrane; it has attached glands, a bilobed prostate and 2 small bulbo-urethral glands.
The penile part, surrounded by spongy erectile tissue, is housed in a ventral groove of the corpus cavernosum, held by a long bulbous-cavity muscle. It ends at the tip of the glans with the urethral process projecting into the urethral fossa.
The penis, cylindrical, highly erectile, extends from the ischial arch (where it is attached by the 2 roots of the corpus cavernosum and the ischio-cavernosum muscles) to below the abdomen, well beyond the testicles. Its free end is enveloped by the erectile tissue of the glans, which is strongly bulging, with a marked crown. It is flanked by two retracting muscles that run up each side of the anus and attach to the sacrum.
The prepuce, a fold of skin not detached from the abdomen, shelters the free part of the penis outside of micturition and erections. It is made up of two folds, an external and an internal one, both of which disappear at the time of erection.
The female reproductive system in the mare
The ovaries (6 to 7 cm long, 3 to 4 m wide) have a marked hilum and globular reliefs, the follicles whose size varies according to the cycle. They are located in the sub-lumbar region.
The uterine tubes, very flexuous and of reduced calibre, open with a very fringed pinnae which covers the ovary and end with a small tubercle at the top of the uterine horns.
The uterus, composed of a short body, is prolonged towards the front by 2 regularly cylindrical and curved horns; towards the rear, it ends in a narrowed neck with a very thick wall. The broad ligament which suspends it splits forward to support the ovary and the uterine tube, thus providing an ovarian bursa.
The vagina, which is very dilatable, bears the raised end of the cervix or “blossoming flower” at the bottom of its cavity.
The vestibule of the vagina (12 to 15 cm long) is separated from the vagina only by a rudimentary hymen and opens through the vulva. On its floor, behind the hymen, is the urinary meatus covered by a mucous fold. Its lateral walls lined with constrictor muscles have an erectile vestibular bulb. The vulva is narrowly slit, limited by thick but simple labia, meeting at a rounded ventral commissure.
The clitoris (7 to 9 cm long) is attached to the ischiatic arch. Its large glans is visible inside the ventral commissure of the vulva, enclosed in a mucous cap.
The mammaries, 2 in number, placed in the inguinal region, are not very large and each bears a short mammary papilla, pierced by 2 or 3 papillary orifices.
The fetal appendages
The innermost envelope, the amnion, is entirely separated from the outermost, the chorion, by an allantois.
The chorion in contact with the uterine wall has an outer surface covered with small reddish placental villi and a cloudy liquid, the uterine milk. The placenta is of the most primitive type, diffuse and epithelial-chorial. From the inner surface, the chorionic plates or “hippomanes” (4 to 5 cm in diameter) are detached and float in the allantoic fluid.
The allantois, in continuity with the future bladder through the canal of the urachus, has a cavity containing an alkaline liquid, initially transparent then amber yellow. This is the first water sac (8 to 10 1 at the end of gestation).
The amnion, a transparent sac, connects to the umbilicus and constitutes the second water sac (at term: 3 to 51).
The umbilical cord (60 cm, 4 cm in diameter) comprises 2 parts (amniotic and allantoic), strongly twisted. A yolk sac persists in the form of a narrow brown cord.
Circulatory and nervous systems of the horse
These systems do not really have any characteristics specific to the species. The arteries have a considerable calibre in relation to the mass of the animal. The veins show a marked superficial venous network, particularly clear under the skin of blood animals.
The lymphatic system is remarkably well developed, both in its network and in the number and mass of lymph nodes. These have a fixed topography and can be palpated on the live animal in specific areas. Rectal exploration can also be used to examine those at the entrance to the pelvis.
The nervous system has a central part weighing around 500-800 g, with the spinal cord accounting for 35% of this weight. The spinal cord occupies only about 4/5 of the vertebral canal.
For more information: horse reproduction, functioning and detection
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