150 DIGESTION. [CHAP. xxn. s 



habits of the animals, or in the nature of their food ; and also according to 

 the complexity of organization exhibited by them. 



In Mammalia, modifications occur in the masticatory process ; the vegetable 

 feeder requiring a more complicated dental apparatus ; the carnivora being 

 provided with teeth of a simpler construction, but more fitted for seizing and 

 lacerating the prey. In others, again, the teeth are adapted to feed on insects, 

 the Insectivora ; in others, the Kodentia, certain teeth are constructed for 

 gnawing dry and resisting substances ; whilst in some of the whales, there are 

 no teeth at all, strictly speaking, but only an apparatus which will allow of 

 retaining the finer kinds of food in their passage into the mouth of the animal. 

 The other sub-processes of digestion are carried on very much upon the same 

 plan as in the human subject, with only such variations as the habits of life 

 of the animals may render necessary. Thus, in a large tribe of Mammals, the 

 Huminantia, the food is macerated in a complex stomach, prior to, as well as 

 after, it has been subjected to a more complete mastication than is employed 

 in any other animals. In these, as well as in all vegetable feeders, the intes- 

 tinal canal is very long and capacious, and the ccecum of great size. In the 

 Carnivora the stomach is simple, and the intestines short and narrow. 



In Birds there are no teeth ; and mastication, properly so called, is effected 

 in the stomach, a portion of which (the gizzard) acquires a great increase of 

 muscular power, and is lined by a dense cuticle, and thus becomes a powerful 

 organ for triturating the food, the bird swallowing pieces of flint or other hard 

 substances to aid the mechanical reduction. Insalivation is but slightly deve- 

 loped, excepting in the woodpecker, where very large salivary glands pour out 

 a considerable quantity of saliva, to aid the bird in picking the dry bark and 

 wood of trees. In some birds, however, a dilated portion of the oesophagus 

 (the crop) gives lodgment to the food for a time, and pours out from its mucous 

 membrane a fluid which probably performs an office similar to that of the saliva, 

 and which at least must serve to moisten the food before it passes further 

 along the digestive tube. Chymification and chylification are essentially the 

 same as in Mammals ; and there are likewise similar differences as regards the 

 length and development of the intestinal tube in carnivorous and herbivorous 

 birds. In Reptiles the digestive process is, on the whole, simpler than in the 

 preceding classes ; but there are the same sub-processes, the alimentary canal 

 being of a simple construction. The dental apparatus varies according to the 

 mode of life of the reptile (the fangs of serpents having evident reference to 

 the predatory habits of that class of reptiles), excepting so far as the beak 

 may be regarded as a substitute ; and in some, as the chelonia, it is entirely 

 absent. In Fishes there are well-developed teeth of various forms, and often 

 very numerous, with a simple stomach and intestine, but no salivary apparatus. 



In the higher Invertebrate, the digestive process is carried on upon the same 

 plan as in the vertebrata. In the Cephalopods, there are powerful instruments 

 of prehension in the arms or tentacles which surround the animal's mouth and 

 head. These animals enjoy a certain power of mastication by teeth, and some 

 of them have a gizzard. All the Mottusca have a large liver ; but other glan- 

 dular organs connected with the intestinal canal, and more or less subservient 

 to digestion, namely, the pancreas and spleen, are absent. The stomach and 

 intestinal tube are very much as in the vertebrata. The articulata have also 

 a digestive system like that in the vertebrate classes ; but the liver is small, 

 and developed in the rudimentary form of ccccal tubes opening into the intes- 

 tine. 



