PRELIMINARY ANATOMICAL AND CHEMICAL DATA 319 



amount of digestion seems to take place, and from 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 alimentary 

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

 In many Arthropods it is possible already to distinguish parts corre- 

 sponding to the stomach, and the small and large intestines 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 few 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 groups 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 carni- 

 vora. 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 i : 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 considerable, in the dog more than half that of the skin, the 

 surface of 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 per- 

 form 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 osso- 

 phagus to the end of the rectum the muscular wall consists, broadly 

 speaking, of an outer coat of longitudinally -arranged fibres, and a 

 thicker inner coat of fibres running circularly or transversely around 

 the tube. Between the layers lies a plexus of non-medullated nerves 

 and nerve-cells (Auerbach's plexus). In the stomach the longitudinal 



