COMPOSITION OF THE SKELETON. 607 



in the interstices of the cartilaginous substance, and, in proportion as 

 these accumulate, additional firmness is bestowed upon the skeleton, 

 until at length it assumes hardness and solidity proportioned to the 

 quantity of 'the contained earthy matter, and becomes converted into 

 perfect bone. 



(1617.) Phenomena precisely similar are observable in tracing the 

 formation and development of the osseous system, even in those genera 

 possessed, when arrived at maturity, of the most completely organized 

 skeletons. 



(1618.) 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. 



(1619.) The complete skeleton of a vertebrate animal may be con- 

 sidered as being composed of several sets of bones employed for very 

 different purposes, consisting of a central portion, the basis and sup- 

 port of the rest, and of various appendages derived from or connected 

 with the central part. The centre of the whole osseous fabric is gene- 

 rally 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 employed in different animals for various 

 and distinct purposes, present 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 ele- 

 ments 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 development of others in particular races that 

 we must ascribe all the endless diversity of form and mechanism so 

 conspicuously met with in this division of the animal world. 



(1620.) 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 of animal organization, it is found to have been 



