Frequency in different animals. It has been found most frequently in the horse, and less so in the dog and ox. Severe exertions conduce to it in horse and dog, while in the ruminant the great bulk of the gastric cavities, covering the whole posterior surface of the diaphragm, tends to prevent protrusion even if a slight rupture has taken place. On the other hand, the weight of the gastric cavities in the ox and the tension upon the œsophagus, when the animals ride each other, sometimes cause laceration of the foramen sinistrum and hernia of the reticulum.
Hernial mass in different animals. In the horse the protruding organ is most commonly the small intestine (enterocele), omentum (epiplocele), colon, or less frequently the cæcum, stomach (gastrocele), or liver (hepatocele). In the ox the reticulum most commonly protrudes, and after that the liver, abomasum, the omentum or the small intestine. In the dog the mass is usually formed by the stomach or liver, or less frequently by the small intestine or omentum.
Causes and mode of formation. The hernia is either congenital or acquired after birth. Again, it may result from imperfect development or from a trauma. Congenital cases usually depend on an arrest of development, the foramen sinistrum fails to close and the abdominal organ passes through by the side of the œsophagus into the chest; or the diaphragm is left imperfect at some other point, and the two cavities, abdominal and thoracic, communicate. In some instances there is left not a distinct opening but a mere relaxation of the parts and under traction, as, for example, by the gullet, an orifice is torn and a hernia protrudes. Such cases are very rare. Other lacerations may be almost all referred to external injuries on the posterior ribs, violent muscular exertions, sudden shocks in connection with falls, or throwing for operations, and overdistension of the abdominal viscera.
1st. External Violence. In the larger animals this may come from blows on the last ribs, by poles or shafts of carriages, the buffers of cars, the end of a gate and the gatepost, projecting ends of bars or beams against which they run, kicks by horse or ox, blows by the horns of cattle or tusks of boars. In the smaller animals in addition to the above, blows with heavy clubs and kicks with heavy boots. In all such cases there is usually a fracture of one or more ribs, the sharp broken ends of which are forced into the diaphragm, which they tear when they again spring outward. The same occurs as the result of falls on hard projecting bodies of any kind.
2nd. Muscular Strain. In heavy draught the fixing of the glottis, ribs and diaphragm and the extreme contraction of the abdominal muscles often lead to extraordinary tension of the muscular septum by the mere violence of which, or when there is superadded a sudden shock, (in plunging in harness, or displacement of the feet or slipping and falling,) the diaphragm is torn, usually in its tendinous portion, and the abdominal viscus protrudes into the chest. If the diaphragm is momentarily relaxed the lesion may take place in the peripheral muscular portion. In still other cases the lesion is at one of the natural openings. The same accident occurs in animals thrown for operation, the abdominal viscera being full, the hind limbs drawn forward so as to further compress the belly, and the muscles being subjected to violent contraction in the efforts to get loose. Violent straining in dystokia is another cause which, however, usually partakes of a sudden shock on the diaphragm when a violent pain sets in.
3rd. Sudden Shocks on the diaphragm, and pressure by the abdominal organs. In the horse especially the weight of the abdominal viscera is very great and the floor of the abdomen inclines downward and forward so that the whole mass presses with great force against the concave diaphragm. In cattle the great weight of the stomach and liver is especially important and in dog and pig of the liver mainly. In trotting or galloping down hill or jumping from a higher to a lower level, or in slipping back on the fore feet so that the horse falls on the knees or shoulders, this pressure is suddenly greatly enhanced and the tense diaphragm may give way in its tendinous portion or the relaxed organ through its muscular tissue. A similar danger attends on the violent straining which attends on difficult cases of parturition, and even in cases of overloaded stomach and tympany and especially when the tortured animal throws itself suddenly and recklessly on the ground. In cattle and sheep this is usually the result of tympany of the rumen, and in solipeds of stomach and intestines.
Rupture by simple overloading or overdistention is, however, a rare occurrence, and many cases attributed to this are in reality instances of post mortem lesions, to be identified by the seat of the laceration in the muscular portion, and by the absence of blood clots, exudate, thickening or other sign of inflammation on the torn border.
Symptoms. These bear a direct relation to the size of the laceration and the mass of abdominal organs that protrude into the chest.
In very grave, recent cases, with a great phrenic rupture and a most extensive protrusion of abdominal organs into the chest, there may be simply the indications of extreme dyspnœa, nostrils and chest widely and persistently distended, nasal mucosa darkly congested, countenance pinched, eyes protruding and fixed, pupils dilated, breathing rapid, shallow and oppressed, and in a few minutes the animal staggers and falls in the death agony.
In cases which are less rapidly fatal, the patient lasting for hours or even days, there occurs, after the accident, deep, difficult and oppressed breathing, but not so violent as to threaten instant suffocation, or more commonly, these symptoms increase slowly as more and more of the hernial mass protrudes through the narrow opening into the chest. This form is usually seen only in animals of a specially quiet disposition, and which have not been subjected to active exertion or excitement after the accident and the hernia has increased by slow degrees only. The patient becomes listless, or very restless, paws, looks at his flanks, shifts from one hind foot to the other or even kicks at the abdomen, lies down carefully, rolls, sits on his haunches (though no more than in other forms of colic), and manifests the anxious, pinched, colic-countenance. The advance of the pain is constant but slow, and usually it is not characterized by that intensity which drives the animal to throw himself down recklessly and to roll and kick with violence. There is also usually an absence of the weak running down pulse of hemorrhagic congestion (thrombosis) and of the pallor of the surface mucosæ which usually attend on the extensive blood extravasations of that disease. An exception may be made in those cases in which the hernial mass is strangulated, as these may closely resemble spasmodic colic or hemorrhagic congestion.