DIGESTIVE SYSTEM 137 



two fluids being the secretions of the liver and the pancreas. 

 Their principal action is to convert its soluble parts into 

 " peptones," which are to be conveyed into the Lymphatic System, 

 and so into the Blood. Their absorption as chjde is eftected by 

 numerous " villi " or projections which line the walls of the whole 

 Canal from the pylorus to the cloaca. At the beginning of the 

 rectum the cseca, when such are functional, receive the remaining 

 chyme, and it is probable that in them certain hitherto undissolved 

 matter, as cellulose and possibly chitin, is acted upon by marsh- 

 gas, so as to extract as much nutrition as possible from the 

 food. After remaining a due time in the caeca, their contents 

 return into the rectum, and are finally ejected through the cloaca 

 as fseces. 



The walls of the Alimentary Canal are composed oi fiwc layers, 

 of which the innermost only is of " endodermal " origin, the rest 

 being "mesodermal" (see Embryology). These layers are: (1) 

 the tunica serosa or adventitia, which is outermost and consists of 

 partly elastic connective tissue ; (2) a layer of smooth musculai 

 fibres, transversely or circularly arranged ; (3) one of smooth 

 muscular fibres, longitudinally arranged ; (4) the tunica submucosa 

 of loose connective tissue, which contains nerves, blood, and 

 lymphatic vessels ; and (5) the tvnica mitcosa or innermost lining, 

 composed of epithelial cells, which give rise to mucous and various 

 specific digestive glands. It is noteworthy that Birds and Reptiles 

 differ from Mammals in the succession of the two muscular layers 

 (2 and 3), since in the last the circular fibres are placed on the 

 inside, next to the submucosa (4), while the longitudinal fibres 

 together with the serosa (1) form the outer wall. These layers 

 vary considerably in the different parts of the Alimentary Canal ; 

 thus the thickening of the walls of the gizzard is due to the 

 excessive development of the muscular layers, while in the 

 oesophagus the mucosa is represented chiefly by ordinary epithelial 

 cells, comparatively few of which form simple mucous glands, 

 though in the region of the proventriculus its cells are transformed 

 into large glands, often closely packed and compressed, constituting 

 the greater part of the thickened walls. Again, in the gizzard no 

 such specific, but only mucous glands occur, the hardened secretion 

 of which invests its cavity with an additional cuticular lining. 

 Both the small and large intestines are characterized by numerous 

 villi, protruding into the canal as excrescences of the two innermost 

 layers, and absorbing the prepared nutritive fluid. Beside the 

 ordinaiy mucous glands the mucosa gives rise to two masses of 

 specific nature which as LiVER and Pancreas grow out of the 

 walls of the duodenum, and thus indicate their point of origin only 

 by their respective ducts. 



The intestine, or gut proper, begins at the pyloric end of the 



