324 



COMPARATIVE PHYSIOLOGY. 



what left out of the case, do not go to the bottom of the matter. 

 Amceba and kindred organisms do not digest themselves. 

 Some believe that the little pulsatile vacuoles of the Infusorians 

 are a sort of temporary digestive cavities. 



But, to one who sees in the light of evolution, it must be 

 clear that a structure could not have been evolved that would 

 be self-destructive. 



The difBculty here is that which lies at the very basis of all 

 life. We might ask. Why do living things live, since they are 

 constantly threatened with destruction from within as from 

 without ? Why do not the liver, kidney, and other glands that 

 secrete noxious substances, poison themselves ? We can not 

 in detail explain these things ; but we wish to make it clear 

 that the difficulty as regards the stomach is not peculiar to that 

 gland, and that even from the ordinary point of view it has 

 been exaggerated. 



Comparative, — More careful examination of the stomachs of 

 some mammals has revealed the fact that in several animals, 



in which the stomaxsh appears to 

 be simple, it is in reality com- 

 pound. There are different 

 grades, however, which may be 

 regarded as transition forms be- 

 tween the true simple stomach 

 and that highly compound form 

 of the organ met with in the 

 ruminants. 



It has been shown recently 

 that the stomach of the hog has 

 an oesophageal dilatation ; and 

 that the entire organ may be 

 divided into several zones with 

 different kinds of glandular epi- 

 thelium, etc. These portions 

 differ in digestive power, in the characteristics of the fluid se- 

 creted, and other details beyond those which a superficial exam- 

 ination of this organ would lead one to suspect. 



The stomach of the horse represents a more advanced form 

 of compound stomach than that of the hog, which is not evi- 

 dent, however, until its glandular structure is examined closely. 

 The entire left portion of the stomach represents an oesophageal 

 dilatation lined with an epithelium that closely resembles that 



Fia. 366 



-Interior of horse^s Btomach 

 (after Chauveau). A. left sac; B, 

 right eac; C, duodenal dilatation. 



