That the external causes cited (cold, excessive heat, stormy weather, etc.) may affect different animals differently and unfavourably is beyond doubt. But the temporary morbid condition of the animal itself is the essential condition to the development of indigestion.
In all probability the animal has in every case been more or less unwell, except in those attacks of indigestion resulting from progressive poisoning during the course of a meal, such as occur when toxic plants like belladonna, veratrine, colchicum, poppies, tobacco, hemlock, etc., have been eaten. In such temporary abnormal states movement of the rumen is partly abolished, or at least is markedly retarded, and, as a consequence of vaso-motor disturbance, the mucous membrane is probably not so abundantly covered with mucus nor so freely irrigated with secretion, as usual. Under these conditions, if the animal, which may appear perfectly well, is allowed to partake of soft, wet, fermentescible food, gaseous indigestion is very likely to develop.
Cultivated grasses, like lucern, sainfoin, clover, and especially grasses grown on artificially manured fields, are regarded as particularly liable to cause gaseous indigestion. This conclusion seems justified by experience, particularly by the fact that young shoots or young, tender after-growths are very liable to fermentation.
This exaggerated tendency to fermentation of tender grasses has even been held exclusively responsible for indigestion, and the cessation of peristalsis in the rumen has been considered a secondary phenomenon, due to distension.
Whether atony of the rumen be the primary condition and abnormal fermentation secondary or inversely, whether fermentation be primary and atony secondary, is not of importance; for either view may be adopted without altering the results, and without the theory being invalidated by the objection that other animals subjected to similar influences had not contracted the condition.
We have already drawn attention to the importance of the condition of the animal’s health for the time being. Digestive peristalsis being diminished, eructation, admixture of food in the rumen, and its onward movement being impeded, fermentation proceeds rapidly. As a consequence the rumen becomes distended, and, cause and effect changing places, the distension in its turn arrests peristalsis, which had previously only been checked.
Local chills, produced by ingestion of food covered with rime, hoarfrost, or simply with dew, may favour gaseous indigestion; such conditions retard or suspend the peristaltic movements by direct local action, and probably by producing vaso-motor disturbance of the mucous membrane. In very rare cases chill has an undeniable influence, either by provoking general vaso-motor disturbance, which reacts on the secretions, or neuro-motor trouble. Gaseous indigestion is not uncommon in animals living on dry winter food, which have been moved from their ordinary quarters and sent on railway journeys or to fairs, etc. As a general rule this form of indigestion is commonest in spring, when the transition from dry winter food to grass, etc., has not been carefully effected. It is also frequent during stormy weather in full summer. Marked barometric changes seem to have an influence on the general health, and particularly on the nervous system, thus favouring organic fermentations.
Symptoms. The earlier symptoms of indigestion escape observation, but they soon begin to develop rapidly, and are then very easy to follow. They always exhibit the same characters, developing, however, with more or less rapidity in different cases. Soon after they commence feeding animals appear to experience special discomfort, which causes those at grass to stop grazing; even when stabled they stop feeding. From this time they show eructation, repeated yawning, restlessness, and some anxiety.
In a quarter of an hour, or less, the left flank begins to project, both laterally and vertically, so that eventually the walls of that part of the abdomen may project above the transverse processes of the lumbar vertebræ. The right flank also becomes swollen, as a consequence of the intestine being thrust out of position. The animal very rapidly shows general disturbance; the nostrils are dilated, the mucous membranes congested, respiration becomes rapid, and asphyxia threatens. The respiration soon becomes panting, for the distended rumen paralyses the diaphragm and compresses the lungs. To ease respiration the animals open the mouth, extend the neck, and stand with the front limbs spread apart; but this fails to prevent dyspnœa becoming more intense and asphyxia imminent.
The heart beats more rapidly, the superficial veins appear swollen, and the mucous membranes cyanotic. The rhythmic contractions of the rumen can no longer be detected by manual examination of the left flank; and on auscultation one neither hears the liquid nor the rolling sound, but only exaggerated crepitation. Finally, there is marked tympanitic resonance on percussion.