GENERAL STRUCTURE OF VERTEBRATA. 
89 
which conducts air into the lungs, p ; these organs, with the 
heart, h, are contained in the portion of the trunk called the 
b si 
Fig. 29 .— Diagram, showing the position of the principal Organs in 
Vertebrata. 
thorax , or chest. At b is seen the position of the brain; and 
at s that of the spinal cord. 
75. The foregoing characters apply, with greater or less 
modification as to details, to the classes of Mammals (com¬ 
monly termed Quadrupeds), Birds, Reptiles, and Fishes; and 
these further agree in the following points, all of which, 
therefore, enter into our idea of a Yertebrated animal. The 
number of limbs or members never exceeds four; and of 
these, two, or even all four, may be absent. In all the 
classes just named, four is the general number; and the 
absence of two or more is the exception. Thus in Mammals, 
we find all four present in every tribe save that of Whales, 
which want the hinder pair; though the upper or anterior 
pair may take the form of arms, wings, legs, or fins, accord¬ 
ing to the element which the animal is formed to inhabit. 
In Birds we find the posterior pair invariably present in the 
form of legs; whilst the anterior pair, though almost always 
developed into wings, is absent in a few instances. In Reptiles 
we find considerable variety; all four members are present in 
the Turtle tribe, and in most Lizards, as well as in the Frog 
tribe; but they are entirely absent in the whole tribe of Ser¬ 
pents ; and there are Lizards which have only one pair. And 
in Fishes, we usually find two pairs, constituting the pectoral 
and ventral fins; but one or both pairs are sometimes absent, 
as in the Eel, Lamprey, &c. We have further to remark, in 
regard to the general characters of Yertebrated animals, that, 
