COMPARATIVE ANATOMY. 
.31 
ral similitude we meet with appropriate differences. The 
valvulce connivcnles , 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.* [PI. XXIII. fig. 2.] 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 op¬ 
position which would arise from pressure, is greater in the 
upright trunk than in any other. That theory therefore is 
pointedly contradicted by the example before us. The 
structure is found where its generation, according to the 
method by which the theorist would have it generated, is 
the most difficult; but ( observe ) 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¬ 
sage from the mouth to the vent. The longest is in the 
deer kind. The intestines of a Canadian stag, four feet 
high, measured ninety-six feet.f The intestines of a sheep, 
unravelled, measures thirty times the length of the body. 
The intestines of a wild cat is only three times the length 
of the body. Universally, where the substance upon which 
the animal feeds is of slow concoction, or yields its chyle 
* It may be questioned, whether these extremely soft rugae 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 lacteals and secreting vessels ; and this appears to be the real U3e 
of the valvules conniventes. — Paxton. 
t Mem. of Acad. Taris, 1701, p. 170. 
