DIGESTION 297 



alimentary tube, which opens into the body- cavity, where a certain 

 amount of digestion seems to take place, and frorn which the food is 

 absorbed either through the cells of the endoderm, or, as in Medusa, 

 by means of fine canals, which radiate from the body-cavity into its 

 walls, and form part of the so-called gastro-vascular system. In the 

 Echinodermata we have a further development, a complete alimen- 

 tary canal with mouth and anus, and entirely shut off from the body- 

 cavity. In many Arthropods it is possible already to distinguish 

 parts corresponding to the stomach, and the small and large intes- 

 tines of higher forms, the digestive glands being represented by 

 organs which in some groups seem to be homologous with the liver, 

 and in others with the salivary glands of the higher Vertebrates. A 

 fe\v Molluscs seem in addition to possess a pancreas. 



Among Vertebrates fishes have the simplest, and birds and 

 mammals the most complicated, alimentary system. In the lowest 

 fishes the stomach is only indicated by a slight widening of the 

 anterior part of the digestive tube. In water-living Vertebrates there 

 are no salivary glands. In birds the oesophagus is generally dilated 

 to form a crop, from which the food passes into a stomach consisting 

 of two parts, one pre-eminently glandular (proventriculus), the other 

 pre-eminently muscular (ventriculus). Among mammals a twofold 

 division of the stomach is distinctly indicated in rodents and cetaceae, 

 but this organ reaches its greatest complexity in ruminants, which 

 possess no fewer than four gastric pouches. The differentiation of 

 the intestine into small and large intestine and rectum is more 

 distinct, both anatomically and functionally, in mammals than in 

 lower forms ; but there are marked differences between the various 

 mammalian goups both in the relative size of the several parts of 

 the digestive tube, and in the proportion between the total length of 

 the alimentary canal and the length of the body. In general, the 

 canal is longest in herbivora, shortest in carnivora. Thus, the ratio 

 between length of body and length of intestine is in the cat i : 4, 

 dog i : 6, man i : 5 or 6, horse 1:12, cow i : 20, sheep i : 27. The 

 relative capacity of the stomach, small intestine, and large intestine, 

 is in the dog 6:2: 1*5, in the horse i : 3^5 : 7, in the cow 7:2:1 

 The area of the mucous surface of the alimentary canal is very con- 

 siderable, in the dog more than half that of the skin, the surface ol 

 the small intestine being three times that of the stomach and four 

 times that of the large intestine. In the horse the mucous surface 

 has twice the area of the skin. 



Anatomy of the Alimentary Canal in Man. The alimentary canal 

 is a muscular tube, which, beginning at the mouth, runs under the 

 various names of pharynx, oesophagus, stomach, small intestine, large 

 intestine, and rectum, till it ends at the anus. Its walls are largely 

 composed of muscular fibres ; its lumen is clad with epithelium, and 

 into it open the ducts of glands, which, morphologically speaking, 

 are involutions or diverticula formed in its course. In virtue of its 

 muscular fibres it is a contractile tube ; in virtue of its epithelial 

 lining and its special glands it is a secreting tube ; in virtue of both 

 it is fitted to perform those mechanical and chemical actions upon 

 the food which are necessary for digestion. Its inner surface is in 

 most parts richly supplied with bloodvessels, and in special regions 

 beset with peculiarly-arranged lymphatics ; by both of these channels 

 the alimentary tube performs its function of absorption. From the 

 beginning of the oesophagus to the end of the rectum the muscular 

 wall consists, broadly speaking, of an outer coat of longitudinally- 



