VERTEBRATA. 311 



ble and consequently feeble condition ; but as greater 

 strength becomes needful, in order to sustain more 

 active and forcible movements, earthy particles are 

 deposited in the interstices of the cartilaginous sub- 

 stance, and as these accumulate, additional firmness 

 is bestowed upon the skeleton, until it becomes con- 

 verted into perfect bone. 



The complete skeleton of a vertebrate animal may 

 be considered as being composed of several sets of 

 bones, employed for 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 

 osseous 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 employed in 

 different animals, for very various and distinct pur- 

 poses, present the greatest 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 which might enter into its composition, 

 does not exist, 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 great division of the animal world. 



The nervous system of the vertebrata consists of 

 the train contained within the cavity of the skull, 

 continued from which, and lodged in a canal formed 

 by the back-bone, is the spinal marrow or spinal cord, 

 whence are derived, at intervals, symmetrical pairs of 

 nerves, which escape from the spinal canal by appro- 

 priate orifices, situated between the different bones of 

 the vertebral column, and are distributed to the volun- 

 tary muscles, and to the integument of the two sides 

 of the body, thus constituting the medium whereby the 

 intimations of the will are communicated to every 

 part, and information received from the external world. 

 The nervous system may thus be compared to an 



