THE FROG 



49 



FIG, 



1 6. Ciliated cells, 

 Rana. 



pharynx of 



c,, cilia ; c,b. t cell body ; ., nucleus. 



body of the cell. The whole pharyngeal region is covered by this 

 ciliated epithelium, and its constituent cells do not behave as 

 isolated units, but act together. Bands of contraction pass over it, 

 producing the same wave-like effect that is seen when the wind 

 blows over a cornfield, only, of course, on a very minute scale. The 

 purpose of this action may 

 readily be ascertained by plac- 11(1111/411 c 

 ing a small piece of cork on 

 the pharynx of a frog immedi- 

 ately after killing ; it will be 

 slowly carried along towards 

 the oesophagus. It will be 

 seen, from the fact that the 

 cilia maintain their lashing in 

 the individual cells when sepa- 

 rated from their fellows, that 



their motion is due to the activity of the protoplasm of the cell, 

 and not to stimuli received from the nerves. 



The transition from pharynx to oesophagus is marked by a 

 change in the character of the mucous membrane, and the single- 

 layered ciliated epithelium gives place to a many-layered compound 

 one. This compound epithelium is in the main similar to that of the 

 skin, its deeper cells are polyhedral and the superficial ones flat and 

 non-ciliated. The membrane is thrown into slight folds running in 

 the longitudinal direction, it is surrounded by connective tissue, 

 and outside this again is a well-developed sheath of muscles, by 

 means of which the action of swallowing is brought about. The 

 whole is covered by the visceral part of the pleuro-peritoneal 

 membrane. 



The change to the stomach, not noticeable externally, is again 

 accompanied by an alterntion in the mucous membrane. The slight 

 folds become very marked and form high ridges (rugee) passing along 

 the organ. The epithelium itself takes on a glandular character, 

 and instead of being a more or less level covering to the folds, it 

 forms a large number of simple or branched tubes, the gastric glands, 

 which sink down into the connective tissue. These do not possess 

 the flask form of the cutaneous glands, but are test-tube shaped, and 

 constitute examples of simple tubular or compound tubular glands, 

 according as to whether they consist of one tube or several opening 

 by a common duct. The cells of the single layer lining the duct are 

 columnar in shape, with more or less clear cytoplasm and a distinct 

 nucleus lying towards their base. In the deeper part, the fundus 

 of the gland, the cells are larger and somewhat cubical. They 

 possess a basally situated nucleus and very granular protoplasm, 



