510 SPECIAL PHYSIOLOGY. 



into contact with the gastric walls; towards the end of digestion, larger 

 quantities pass the pylorus. Whilst the pylorus permits the passage 

 out of the stomach of the pulpy products of gastric digestion, such 

 solid substances as do not yield to the digestive process are not allowed 

 to pass, apparently because they excite the contraction of the circular 

 pyloric muscular fibres. Such substances, as well as fish-bones, but- 

 tons, plum-stones, or other bodies accidentally swallowed, remain in 

 the stomach for some time after the evacuation of its digestible con- 

 tents; but after a certain delay, the pylorus relaxes, and allows them 

 also to pass into the intestinal canal. The movements of the stomach 

 are partly reflex, being excited through the pneumogastric nerves, as is 

 shown by experiments on animals; but it would also seem probable 

 that a direct stimulation of its muscular fibres may co-operate. The 

 sphincter fibres at the cardiac end appear to be under the government 

 of the sympathetic nerves. It is not known whether the contraction 

 of the pylorus is a reflex act. 



The gastric movements aid in the function of digestion, by rotating 

 the food in the stomach, thus exposing all parts of the digesting mass 

 to the action of the gastric fluid, and by continually removing the 

 softer parts from the surface, and expelling them gradually through 

 the pylorus, so that fresh portions of that surface are then exposed. 

 The pressure exercised upon the contents of the stomach, may further 

 assist in the process of venous absorption. It is to be observed, how- 

 ever, that portions of food, placed in perforated metal tubes or balls, 

 and introduced into the stomach, are nevertheless digested. 



Movements of the Intestines. 



The intestinal canal, Fig. 89, d to r, or portion of the alimentary 

 canal extending from the stomach downwards, is divided into a longer 

 and narrower part, called the small intestine, d to i, and a wider and 

 shorter part, named the large intestine, c to r. 



The small intestine extends from the pylorus, p, to a valvular open- 

 ing leading into the large intestine, c ; it measures about 20 feet in 

 length, and becomes somewhat, though slightly, narrower from above 

 downwards. This long tube lies in coils, or convolutions, occupying 

 the middle and lower part of the abdominal cavity, and the pelvis, Fig. 

 13. It is supported by a broad double fold of the peritoneum, named 

 the mesentery, which is attached, by a shorter posterior margin, to the 

 back of the abdomen, but is connected by a longer anterior margin, 

 with the back of the small intestine, so that both it and the intestine 

 are thrown into folds, which are capable of constant change in form 

 and position. The layers of the mesentery are prolonged over the in- 

 testine, and form its outer or serous coat ; and between these two 

 layers, are contained the bloodvessels, lymphatics, and lymphatic 

 glands, and the nerves of the intestine, all of which help to support 

 this part. 



The small intestine commences on the right side of the vertebral 

 column, beneath the right lobe of the liver, and after undergoing its 

 numerous convolutions, terminates in the lower part of the right side 



