'288 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



external to which are submucous and muscular coats, and, in the 

 abdominal organs, there is a thin serous layer outside all. 



The muCOUS membrane consists of epithelium internally, which, as far 

 as the stomach, is stratified squamous, but in that organ and the intestines, 

 simple columnar. Goblet cells are everywhere common, peptic glands are 

 found in the stomach, and ylands of Lieberkiihn throughout the intestines. 

 Each peptic gland opens on the surface of the stomach by a narrow neck, 

 lined with columnar epithelium. It branches at its deep end, and the 

 branches are lined by granular cuboid chief cells, among which are scattered 

 (in the glands of the cardiac end) a much smaller number of ovoid cells. 

 Beneath the epithelium is a layer of connective tissue surrounding the 

 glands, and the external limit of the mucous membrane is marked (in the 

 stomach and intestines) by a very thin sheet of unstriated muscle, the 

 muscularis mucosce. 



The SUbmUCOUS COat is made up of loose connective tissue, traversed 

 by vessels and nerves. 



The muscular COat varies very much in thickness, attaining its maxi- 

 mum in the stomach. It consists of an inner circular and an outer longi- 

 tudinal layer in the gullet and succeeding parts. The fibres are striated in 

 the gullet and pharynx, and the tongue is mainly composed of such fibres ; 

 but the muscle in the walls of the stomach and intestines is made up of 

 unstriated fibres. 



The salivary glands and pancreas consist of a number of lobules 

 united together by connective tissue. Each lobule is composed 

 of numerous acini, blindly ending tubes lined by glandular epi- 

 thelium, from each of which a duct, lined by simple columnar 

 epithelium, proceeds. These ducts unite again and again to 

 form the main duct of the gland. The preceding type of 

 structure is termed racemose. 



The liver, as in the frog and pigeon, is mainly made up of 

 polyhedral, granular, hepatic cells (Fig. 86), placed in the meshes 

 of the network formed by the bile-capillaries, from which the 

 hepatic ducts arise. The hepatic cells are aggregated into small 

 lobules, around each of which is an interlobular capillary network 

 formed by the ultimate branches of the portal vein. From this 

 network veinlets pass into the centre of the lobule, and there 

 unite into a small intralobular vein which carries away its blood. 

 By the union of these small vessels sublobular veins are formed, 

 which are the factors of the hepatic veins. 



The food consists of vegetable substances, and the great length 

 of the alimentary canal is correlated with this, an immense 

 absorptive surface, largely augmented by the caecum with its 

 spiral valve, being given. 



The food is divided by the incisor teeth, and ground up by 



