CHAPTER, VII. 

 DIGESTION. 



52. THE food is received into the digestive or alimentary 

 tube; there it is subjected to a series of agencies by which it 

 is in greater or less part digested or reduced to a condition in 

 which ifc can be sucked up by appropriate vessels ; and, while 

 this portion is absorbed into the circulation, the effete re- 

 mainder passes on and is discharged. The digestive tube, 

 beginning at the mouth, is continued to the stomach by the 

 throat and gullet, while the stomach is succeeded by the 

 small and great intestine. In its passage along this tract, 

 the food is subjected to both mechanical and chemical pro- 

 cesses; and it is proposed to follow its course, and mark the 

 mechanical actions to which it is subjected, before directing 

 attention to the chemical changes. But, among the first of 

 these mechanical actions is that of the teeth; and the whole 

 structure and history of these may be conveniently considered 

 at the outset, previous to noticing their action in mastication. 



53. The Teeth. Under this term, in its widest significa- 

 tion, may be included all hard structures for the trituration 

 of the food; and these are of many different kinds, and found 

 in different positions. In certain molluscs, e.g., the genus 

 Bulla, and in Crustacea, e.g., the lobster and crab, teeth of 

 shell are developed in the walls of the stomach ; and both in 

 Crustacea and insects, modified limbs, called jaws, are used 

 for the seizure of food : in the cuttle fishes, the same function 

 is performed by a horny beak in two pafcs like that of a 

 bird; and in other molluscs by the rasping action of minute 

 plates set on a long muscular tongue. In the vertebrata, the 

 same variety of structures for breaking down the food exists. 

 Birds and turtles have beaks which are horny developments 

 of the integument covering the jaws, and granimiverous 



