Physiological studies on eructation in ruminants

Abstract

(1) The eructation reflex in sheep and cattle, and factors affecting it, have been studied mainly by the method of forced air insufflation and direct recording of intraruminal pressure. (2) Eructation is a complex, co-ordinated reflex involving the rumen, reticulum, cardiac orifice and the oesophagus. It consists essentially in the movement of free gas from the dorsal rumen, forwards and downwards to the cardia. This is accomplished by- (a) A forward wave of contraction of the ruminal musculature (as distinct from the usual backward cycle of contractions). (b) Opening of the cardiac orifice which is brought about by contraction of the lateral and medial pillars of the rumino-reticular fold . (c) The clearing of the cardia of fluid ingesta is affected by relaxation of the reticulum. The relative importance of this phase depends on the degree of filling of the rumen. (3) The main stimulus for eructation is gas pressure in the posterior dorsal sac of the rumen. (4) From variations amongst the individual experimental animals it is believed that the efficiency of the reflex may depend on hereditary factors. (5) The factors affecting the reflex are: - A. Mechanical. (i) Obstruction of the oesophagus. (ii) Frothing of the ingesta. (iii) The degree of filling of the rumen and posture. Overfilling of the rumen was found to hinder eructation by increasing the distance between the free gas and cardiac orifice. Elevation of the hindquarters had a similar effect, while elevation of the forequarters mitigated the effects of overfilling. Animals with a non-functioning reticulum were found to be particularly susceptible to overfilling. B. Alkalosis. In the absence of other factors, the efficiency of eructation was found to vary inversely with the degree of alkalosis as determined by the CO₂-combining power of the blood. This was due mainly to reticular paralysis. C. Abdominal Vagotomy. Section of the right ventral branch of the vagus caused abomasal distension and chronic tympany, again due mainly to inhibition of reticular activity. Section of the left dorsal branch diminished the strength of ruminal contractions and eructation efficiency for the first three weeks with subsequent partial recovery. D. Reflex inhibition from the Posterior Digestive Tract. Distension of both the abomasum and caecum reduced the eructation efficiency again by inhibition of the reticulum. E. The effect of Drugs influencing Ruminal Motility. (i) Hypomotility. (a) Small doses of prussic acid inhibited the reticulum and backward movement of the rumen with consequent inefficiency of eructation and abolition of the reflex. (b) Atropine, histamine and adrenaline were found to inhibit the reflex completely even before a total paralysis of the rumen was induced. (ii) Hypermotility. Carbamylcholine and veratrine both cause spasm of the rumen and reticulum with consequent interference with eructation. Their therapeutic use is contraindicated.