VEKTEBRATA. 479 



of animal organization, it is found to have been constructed upon 

 principles the most aberrant and remote from those which an 

 extensive investigation of the lower animals has revealed to the 

 physiologist. 



(520.) A skeleton, described generally, is made up of the fol- 

 lowing portions : 1st. of a chain of bones, placed in a longitudinal 

 series along the mesial line of the back, and more or less firmly 

 articulated with each other, so as to permit certain degrees of 

 flexure. These bones, examined individually, present various 

 additional parts destined to very different ends : some defend the 

 central axis of the nervous system from external violence ; others, 

 when present, guard and enclose the main blood-vessels ; and the 

 rest, either acting as prominent levers, serve to give insertion to 

 the muscles which move the spine, or afford additional security to 

 the articulations between the vertebral pieces. Those vertebrae 

 which defend the posterior portions of the nervous axis, usually 

 called the spinal core/, constitute the spine ; while those enclosing 

 the anterior extremity of the nervous axis, which, for reasons 

 hereafter to be explained, becomes dilated into large masses 

 forming collectively the brain, is by the human anatomist distin- 

 guished as the cranium or skull. 



Secondly, we find appended to the cranial or cephalic portion 

 of the spine, a set of bones disposed symmetrically, and forming 

 the framework of the face : these bones, it is true, have by many 

 Continental writers been regarded as constituting additional ver- 

 tebrae, the parts of which are still recognizable, although amazingly 

 modified in shape, so as to enclose the different cavities wherein 

 the senses of vision and smell, as well as the organs of mastication, 

 are situated. We shall not, however, waste the time of the stu- 

 dent by considering in this place the as yet unsettled and vague 

 opinions of transcendental anatomists upon this subject ; it is 

 sufficient for our present purpose to indicate the facial bones as 

 appendages to the cranial vertebrae, avoiding for the present fur- 

 ther discussion concerning them. 



Another most important addition to the central axis of the 

 skeleton is obtained by the provision of lateral prolongations, 

 derived from the transverse processes of the vertebrae, which form 

 a series of arches largely developed at certain points, so as more 

 or less completely to embrace the principal viscera, and give 

 extensive attachment to muscles serving for the movements of the 

 body. 



