These terms are used to indicate the escape and lodgment of some of the abdominal contents in the inguinal canal. Although a common form of rupture in man, it is of rare occurrence in horses, owing chiefly to the horizontal position of the abdomen in the latter, and the slope of the belly throwing the weight of the viscera forward towards the diaphragm, instead of bearing down upon the inguinal region, as in the former. This particular rupture, both in man and beast, is more likely to occur in males than females. In geldings the operation of castration, by inducing wasting of the spermatic cord and contraction of the opening by which the belly communicates with the inguinal canal, renders the occurrence much rarer in them than in stallions.
The predisposition to both inguinal and scrotal hernia is no doubt hereditary, and as a congenital condition it is well known to veterinary surgeons and castrators.
The very rare occurrence of this form of hernia in geldings seems to furnish a clue to its more common occurrence in stallions, for while in the former the abdominal ring through which escape of the gut or omentum is effected is very small, in the latter it is very large. This, however, is not the only reason for the greater liability observed in the one as compared with the other. The habitual upright posture which the stallion assumes in the act of copulation, by relaxing the abdominal muscles and causing the intestines to gravitate towards the inguinal region, favours, as in man, their entrance -into the inguinal canal, and especially so when the digestive organs are distended with food. The less frequent causes are severe efforts at draught in deep ground, slipping, rolling, rearing, and kicking.
The inguinal canal being situated in the region of the groin, and practically out of view, the presence of inguinal hernia may be overlooked unless conditions arise in it to provoke pain and draw attention to the affected part. It is by no means a rare occurrence for veterinary surgeons and castrators to find a portion of omentum or "hell", and even a piece of intestine, in the scrotum while removing the testicles, which was not suspected to exist before the purse was laid open; and one or the other of these structures may be lodged in the inguinal canal at the time of operation, and descend and protrude from the scrotal wound after the testicles have been removed and the animal has been allowed to rise. It would appear, therefore, that both inguinal and scrotal hernia may sometimes exist without occasioning symptoms or any inconvenience to the animal whatever. There is no doubt, too, that portions of omentum, and maybe intestine also, which find their way into the inguinal canal during the descent of the testicles, return again spontaneously into the abdominal cavity as the foal acquires strength and age advances.