GENERAL STRUCTURE OF VERTEBRATA. 89 
‘which conducts air into the lungs, p ; these organs, with the 
heart, 2, are contained in the portioy. of the trunk called the 
6 si 
Fig. 29.—Diackam, SHOWING THE POSITION OF THE PRINCIPAL ORGANS IN 
VERTEBRATA. 
_ thorax, or chest. At 6 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 Vertebrated 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 Vertebrated animals, that, 
