478 VERTEBRATA. 



genera possessed, when arrived at maturity, of the most com- 

 pletely organized skeletons. 



In the very young animal the bones consist exclusively of 

 cartilage ; but as growth proceeds, earth becomes deposited by the 

 blood-vessels in the as yet soft and flexible pieces of the skeleton, 

 until by degrees they acquire density and strength as the animal 

 advances towards its adult condition. 



The complete skeleton of a vertebrate animal may be considered 

 as being composed of several sets of bones employed for very 

 different purposes ; consisting of a central portion, the basis and 

 support of the rest, and of various appendages derived from or 

 connected with the central part. The centre of the whole os- 

 seous fabric is generally made up of a series of distinct pieces 

 arranged along the axis of the body, and this part of the skeleton 

 is invariably present ; but the superadded appendages, being em- 

 ployed in different animals for various and distinct purposes, pre- 

 sent the greatest possible diversity of form, and are many of them 

 wanting in any given genus : "so that a really complete skeleton, 

 that is, a skeleton made up of all the pieces or elements which 

 might, philosophically speaking, enter into its composition, does 

 not exist in nature ; inasmuch as it is owing to the deficiency of 

 some portions, and the developement of others in particular races, 

 that we must ascribe all the endless diversity of form and me- 

 chanism so conspicuously met with in this division of the animal 

 world. 



Nevertheless, although there is no such a thing in Creation as 

 a fully developed skeleton, it will be necessary, in order to prepare 

 the student for the contemplation of the numerous modifications 

 met with in this portion of the animal economy, hereafter to be 

 described, briefly to enumerate the component parts which might 

 theoretically be supposed to enter into the construction of the 

 framework of an animal ; and thus by comparison he will be 

 enabled, as we proceed, to appreciate more readily the variations 

 from a general type apparent throughout the vertebrate classes. 

 It may, likewise, be as well thus early to caution the anatomist 

 who has confined his studies to the contemplation of the human 

 body, against taking the skeleton of Man as a standard whereby 

 to direct his judgment; for Man, so highly raised by his 

 intelligence and mental powers above all other beings, is, so 

 to speak, a monstrosity in the creation ; and, so far from 

 finding in the human frame the means of elucidating the laws 



