OSSEOUS SYSTEM OF MAMMALIA. 2 



the organs of animal life and of the senses ; yet, if we compare the 

 fish, the reptile, the bird, and the mammal, so great is the variation 

 of form between them, that it is only by an analysis of the parts of 

 this framework, and a consideration of its essentials, that we become con- 

 vinced that the laws of simple modification have not been deviated from. 

 The presence of an internal skeleton is peculiar to the vertebrata, and 

 two parts of this skeleton are essential, as connected with the great cen- 

 tres of the nervous system, the brain, and spinal chord, which they en- 

 close and protect : these two parts are the skull and the vertebral 

 column :* the other parts are their accessories, and may be present or 

 absent as the conditions of existence render necessary : these acces- 

 sories are the ribs and the limbs, or extremities. The skull encloses 

 the brain, and in the cavities of its facial portion lodge three of the organs 

 of the senses ; viz., those of sight, taste, and smell. The organs of 

 hearing are in the cranial portion : the sense of feeling is more or less 

 universal over the body. 



The vertebral column adjoins the skull, and consists of numerous dis- 

 tinct bones, by the due junction of which a canal is formed, enclosing 

 that prolongation of the brain, termed the spinal chord. The structure of 

 the vertebral column allows it considerable flexibility of motion, and in 

 many animals it is carried out, by the addition of supplemental bones, 

 into a tail. 



The ribs, attached to the vertebral column posteriorly, and anteriorly to 

 the sternum, or breast-bone (when this is present), encircle the organs of 

 animal life the heart and the thoracic viscera. They are seldom wanting, 

 but their number is subject to great variation ; in some Snakes they amount 

 to hundreds. The limbs are never more than four in number : they are, 

 however, subject to the utmost modifications of form ; they range from 

 the hand of Man to the solidungulous foot of the Horse, or the paddle 

 of the Whale from the wing of the bird to the fin of the fish through a 

 thousand gradations ; yet ever preserving their intrinsic characters. In 

 some, as in the cetaceous mammals, one pair is wanting, and this is 

 always the posterior ; in others, as Serpents, either both pairs are deficient 

 or, as in the Boa and Python, one pair, the posterior, exists in a rudi- 

 mentary condition, but not destitute of utility. 



The following sketches may be taken as representations of the 

 osseous framework, as modified in Mammalia, birds, reptiles,-)- and fishes. 

 In each example, the skull encases the brain ; and the vertebral column, 

 the spinal chord. The organs of sight, smell, taste, and hearing, have, 



* The skull itself is to be regarded as consisting, elementarily, of vertebras modified in their parts, 

 so as to become a case for the brain. 



t The reptiles of Cuvier are now usually divided into Reptilia and Amphibia; the latter correspond- 

 ing to his order of " Batraciens." 



