OF DIGESTION. 2-31 



tion.* Dr. Haighton observed the same hour-glass contraction 

 in a living dog, and remarked the peristaltic motion to be much 

 more vigorous in the pyloric half.f 



During digestion, the contents of the stomach acquire an acid 

 of a volatile nature, and, on exposure to the air, one of a more 

 fixed kind, probably the phosphoric. 



In granivorous birds the food passes into the crop, and from 

 this into a second cavity from which it enters the gizzard, — a 

 strong muscular receptacle, lined by a thick membrane, in which, 

 instead of having been masticated, it is ground by means of 

 pebbles swallowed instinctively by the animal. Some gramini- 

 vorous quadrupeds with divided hoofs have four stomachs, into 

 the first of which the food passes when swallowed, and from this 

 into the second. It is subsequently returned by portions into 

 the mouth, chewed, and again swallowed, when, by a contraction 

 of the openings of the two first stomachs, it passes over them 

 into the third, and from this goes to the fourth. Some birds and 

 insects also ruminate. The stomachs of some insects and Crus- 

 tacea contain teeth. Some zoophytes are little more than a sto- 

 mach : others have several openings on the surface leading by 

 canals that unite and run to the stomach, — a structure called by 

 Cuvier, mouth-root. Between the most distinct kinds of stomach 

 we see numerous intermediate varieties. 



Vomiting cannot occur unless the stomach has the resist- 

 ance of the diaphragm and abdominal muscles, or of something in 

 their stead. Different persons have made the horrid experiment 

 of giving an emetic to an animal and, after the abdominal mus- 

 cles were cut away, observing how fruitless were all the efforts 

 of the stomach to reject its contents till they applied their hands 

 in place of these muscles, when the stomach, being forced 

 by the diaphragm against the resistance, instantly accomplished 



* Phil. rram. 1808. 



f Transaction* of the Medical Society of London. Vol.2. 1788. 



