DIGESTIVE SYSTEM 137 
two fiuids being the secretions of the liver and the pancreas. 
Their principal action is to convert its soluble parts into 
“eptones,” which are to be conveyed into the Lymphatic System, 
and so into the BLoop. Their absorption as chyle is effected 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 ceca, 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 ceca, their contents 
return into the rectum, and are finally ejected through the cloaca 
as fxces. 
The walls of the Alimentary Canal are composed of five 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 muscular 
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 tunica mucosa 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 
cesophagus the mwcosa 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 
ordinary 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 
