THE FROG 51 



longitudinal muscles. The whole, like the oesophagus, is enclosed 

 in a portion of the pleuro-peritoneal membrane, in this case it is 

 the peritoneum itself. 



The end of the stomach is marked by the pylorus, at which point 

 the long folds of the mucous membrane disappear, and the outer 

 band of circular muscles becomes greatly thickened. This ring of 

 muscles, a sphincter, enables the stomach to be cut off almost, if 

 not entirely, from the intestine. The first part of the intestine is the 

 duodenum, into which the bile and pancreatic fluid are passed. No 

 tabular glands are present in its mucous membrane, which is not 

 thrown into folds, but is raised into a number of small conical 

 projections, papillae, irregularly distributed. In the next part of the 

 intestine, the ileum, the longitudinal folds again make their appear- 

 ance and are closely set together. The rectum is devoid of papillae 

 or folds, and covered with a plain epithelium. The mucous mem- 

 brane of the entire intestine consists of a columnar epithelium, whose 

 cells have distinct, basally situated nuclei, and rest upon a basement 

 membrane. A large number of glandular cells are to be found 

 among the apparently unspecialised epithelial cells. These cells, 

 called goblet cells, from the fact that they often contain a large drop 

 of secretion at the outer end, and hence present a somewhat fanciful 

 resemblance to a globet, do not appear to be aggregated into 

 definite glandular areas as in the frog, as some of them are in the 

 rabbit. The remaining layers of the intestinal wall are similar to 

 those in the stomach, save that the muscularis mucosae is so thin 

 that it is often overlooked, and the sub-mucosa is very plentifully 

 supplied with blood-vessels. The whole is encased in peritoneum. 

 During the time that the food is undergoing digestion a slow move- 

 ment of the intestinal walls is kept up. It takes the form of a series 

 of waves of contraction, which slowly pass from pylorus to rectum, so 

 assisting the digestive juices to mix with the food and at the same 

 time gradually passing it onwards. As has been previously noted, 

 this movement is termed peristalsis, and is brought about by the 

 involuntary muscles of the muscular coats. 



The pancreas furnishes an example of a complex gland, and 

 is more intricate than the branched glands of the stomach. A section 

 shows that it is composed of lobules, each consisting of a group of 

 large granular cubical cells around a small central cavity or lumen. 

 The cavities or alveoli of neighbouring lobules open into a small 

 tube or duct, and these ducts join with one another and ultimately 

 open into the bile duct. The ducts are readily detected in a section, 

 owing to their comparatively large lumen and the flat cells forming 

 their walls. It will be seen then that such a gland bears some 

 resemblance to a bunch of grapes squeezed together, the ducts 



