280 A MANUAL OF PHYSIOLOGY 



Comparative. In the lowest kinds of animals, such as the 

 Amoeba, there is neither mouth, nor alimentary canal, nor anus : the 

 food, wrapped round by pseudopodia, is taken in at any part of the 

 animal with which it happens to come in contact. A vacuole is 

 formed around it. Acid is secreted into the vacuole, the food is 

 digested within the cell-substance, and the part of it which is useless 

 for nutrition is cast out again at any part of the surface. 



Coming a little higher, we find in the Coelenterates a mouth and 

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

 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 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 

 few Molluscs seem in addition to possess a pancreas. 



Among Vertebrates fishes have the simplest, and birds and mam- 

 mals 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 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 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 



