AMPHIBIA. 193 



mucosce ; (b) a layer of connective tissue with glands, blood- 

 vessels, and lymphatics; (c) a layer of simple columnar epi- 

 thelium, adjoining the cavity of the gut. 



The epithelium is of most interest. It is ciliated in the 

 oesophagus and beginning of the stomach, and many of its cells, 

 known from their shape as goblet-cells, are unicellular mucus- 

 secreting glands. Besides this there are tubular multicellular 

 glands lined by the epithelium and lying in the connective- 

 tissue layer of the mucous membrane. They are of three 

 kinds : a, cesophageal glands in the gullet ; j3, peptic or gastric 

 glands in the stomach; -y, glands of Lieberkiihn in the small 

 intestine. 



The deeper parts of the peptic glands are lined by cuboidal 

 cells, which secrete the gastric juice. 



There are several points of interest in the histology of the 

 mucous membrane lining the mouth. Its epithelium is strati- 

 fied near the margins, elsewhere simple, columnar, and ciliated. 

 Numerous goblet-cells are present. The tubules of the inter- 

 maxillary glands are lined by mucus-secreting epithelium. 



The teeth are developed in the mucous membrane of the mouth. 

 The projecting part or crown, which is forked, is separated by a 

 constriction from the rest of the tooth, or socket, which contains 

 a pulp-cavity, in which is the pulp, a small mass of vascular 

 connective tissue. The tooth is mostly made up of dentine, the 

 constituents of which are wavy tubules filled by prolongations of 

 the pulp. The crown is covered by a cap of extremely hard 

 enamel, exhibiting a layered structure, and containing prolon- 

 gations of the dentine tubules. It is secreted by the epithelium 

 of the mouth. The socket is covered by a layer of cement, 

 resembling bone in structure. The teeth are all very similar, 

 and during life are constantly replaced by new ones, which grow 

 up from their bases. 



The liver (cf. Fig. 51) is mostly made up of polyhedral glan- 

 dular hepatic cells, granular, nucleated, and containing fat drops. 

 Between these cells minute tubes, the bile-capillaries, run, forming 

 a complex network from which the ultimate branches of the 

 hepatic ducts arise. The vessels supplying the liver with blood 

 break up into capillaries within it, and in these the branches of 

 the hepatic veins, which carry blood from the liver, take their 



