GENERAL STRUCTURE OP VERTEBRATA. 89 



which conducts air into the lungs, p ; these organs, with the 

 heart, h, are contained in the portioy. of the trunk called the 



s I 



Fig. 29. DIAGRAM, SHOWING THK POSITION op 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 Eeptiles 

 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, 



