132 COMPARATIVE ANATOMY. 



deglution of animals, which thrust their food up instead of 

 duicn^ and also through a longer passage, a proportionably 

 more powerful apparatus of muscles is provided ; more pow- 

 erful, not merely by the strength of the fibres, which might 

 be attributed to the greater exercise of their force, but in 

 their collocation, which is a determinate circumstance, and 

 must have been original. 



IV. The gullet leads to the intestines ; here, likewise, 

 as before, comparing quadrupeds with man, under a gener- 

 al similitude we meet with appropriate differences. The 

 valvule conniventes, or, as they are by some called, the 

 semilunar valves, found in the human intestine, are want- 

 ing in that of brutes. These are wrinkles or plaits of the 

 innermost coat of the guts, the effect of which, is to retard 

 the progress of the food through the alimentary canal. It 

 is easy to understand how much more necessary such a 

 provision may be to the body of an animal of an erect pos- 

 ture, and in which, consequently, the weight of the food 

 is added to the action of the intestine, than in that of a 

 quadruped, in which the course of the food, from its en- 

 trance to its exit, is nearly horizontal ; but it is impossible 

 to assign any cause, except the final cause, for this distinc- 

 tion actually taking place.* So far as depends upon the 

 action of the part, this structure was more to be expected 

 in a quadruped than in a man. In truth, it must, in both, 

 have been formed, not by action, but in direct opposition 

 to action, and to pressure ; but the opposition, which would 

 arise from pressure, is greater in the upright trunk than in 

 any other. That theory, therefore, is pointedly contradict- 

 ed by the example before us. The structure is found, where 

 its generation, according to the method by which the theo- 

 rist would have it generated, is the most difficult ; but {ob- 

 serve) it is found, where its effect is most useful. 



The different length of the intestines in carnivorous and 

 herbivorous animals has been noticed on a former occasion. 

 The shortest, I believe, is that of some birds of prey, in 

 which the intestinal canal is little more than a straight pas- 



* It may be questioned, whether these extremely soft rugaj or folds 

 of the villous coat of the intestine can in the least retard the passage of 

 the food through its canal ; nor does the erect attitude of man require 

 them ; for since there are as many of the convolutions of the intestines 

 ascending as there are descending, the weight of the food can have 

 no Influence in the action of the intestine : it is certain, however, that 

 this arrangement of the internal coat, affords a more extensive surface 

 for the lacteah and secreting vessels ; and this appears to be the real 

 use of the valvules conniventes. Paxton. 



