340 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the stomach. The muscular movements of the stomach are not so regu- 

 larly peristaltic as in the intestines, on account of the want of symmetry 

 of the direction of the muscular fibres. 



The nervous mechanism of the gastric movements has not been 

 thoroughly cleared up. Numerous nervous ganglia have been found in the 

 walls of the stomach, and this fact, in addition to the observation which 

 has often been made, that these movements may occur in a stomach which 

 has been separated from the central nervous system, would indicate that 

 the ganglia start up these movements in this organ. Nevertheless, the 

 movements of the stomach are dependent on and governed by the central 

 nervous symptom, since the movements of the stomach may be induced 

 by stimulation of the peripheral ends of the pneumogastric nerves when 

 the stomach is full. That the movements of the stomach are not, how- 

 ever, solely dependent on impulses coming through the pneumogastrics is 

 proved by their occurrence after these nerves have been divided. Stimu- 

 lation of the sympathetic and cceliac plexus is said to evoke contractions 

 of the gastric walls, probably through changes in the blood supply of the 

 stomach. The vagus nerve is without doubt the principal path through 

 which the movements of the stomach are controlled by the central 

 nervous system. 



The contractions of the muscular walls of the stomach not only 

 serve to mix the food contained in this organ, but also to bring all the 

 contents of the stomach in contact with the secreting portion of its 

 walls. This is especially important in such animals as the horse, where 

 only one-half the organ is thus active. 



The oesophageal sphincter is always tightly closed, especially in 

 horses, where it is very firmly constricted, and so prevents the return of 

 the food to the mouth, even when the stomach is strongly compressed. 

 The pyloric sphincter is not so powerful and its contractions are not so 

 prominent, but are, nevertheless, well marked in the carnivora. the pig, 

 and the ruminant. 



In solipedes the contraction of the pylorus is only faintly marked, 

 and, as a consequence, the opening of the stomach into the intestines in 

 these animals, for reasons which will be given directly, is nearly always 

 patulous. Oser has found that stimulation of the pneumogastric nerves 

 in the neck leads to an immediate contraction of the pylorus, the intensity 

 and duration of the contraction depending on the degree of stimulation. 

 Stimulation of the thoracic portion of the sympathetic nerve arrests the 

 spontaneous contractions of the pylorus, the influence of this stimulation 

 being progressive, attaining its maximum in one or two minutes and 

 then slowly declining. After the conclusion of the stimulation the 

 spontaneous contractions, which exist even after section of the va<tf and 

 splanchnic nerves, at first are feeble, and attain their normal degree in 



