332 COMPARATIVE PHYSIOLOGY. 



covering (capsule) in structure allied to the serous cohering of 

 the stomach and intestines. 



Details will be referred to in various parts of the sections 

 devoted to this subject as far as may be necessary to render 

 function clear, but we think these few generalizations may tend 

 to widen the student's field of view, and at the same time lessen 

 his labor and render it more effective. 



THE MOVEMENTS OF THE DIGESTIVE ORGANS. 



As with other parts of the body, so in the alimentary tract, 

 the slower kind of movement is carried out by plain muscular 

 fibers ; and the movements, as a whole, belong to the class 

 known as peristaltic ; in fact, it is only at the beginning of the 

 digestive tract that voluntary (striped) muscle is to be found 

 and to a limited extent in the part next to this — i. e., in the 

 oesophagus. 



Teeth in the highly organized mammal are remarkable in 

 being to the least degree living structures of any in the entire 

 animal, thus being in marked contrast to other organs. The 

 enamel covering their exposed surfaces is the hardest of all the 

 tissues, and is necessarily of low vitality. We have already 

 alluded to the difference in the teeth of different animals, and 

 their relation to customary food and digestive functions. In 

 fact, it is clear that the teeth and all the parts of the digestive 

 system are correlated to one another. The compound stomach 

 of the ruminants, with its slow digestion of a bulky mass of 

 food which must be softened and thoroughly masticated be- 

 fore the digestive juices can attack it successfully, harmonizes 

 with the powerful jaws, strong muscles of mastication, and 

 grinding teeth ; and all these in marked contrast with the teeth 

 of a carnivorous animal with its simple but highly effective 

 stomach. Compare figures in earlier pages. 



Mastication in man is of that intermediate character befit- 

 ting an omnivorous animal. The jaws have a lateral and for- 

 ward-and-backward movement, as well as a vertical one, though 

 the latter is predominant. The upper jaw is like a fixed mill- 

 stone, against which the lower jaw works as a nether millstone. 

 The elevation of the jaw is effected by the masseter, temporal, 

 and internal pterygoid muscles ; depressed by the mylohyoid 

 and geniohyoid, though principally by the digastric. The jaw 

 is advanced by the external pterygoids ; unilateral contraction 



