608 VERTEBRATA. 



constructed upon principles the most aberrant and remote from those 

 which an extensive investigation of the lower animals has revealed to 

 the physiologist. 



(1621.) A skeleton, described generally, is made up of the following 

 portions : first, 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, acting as prominent levers, either serve to 

 give insertion to the muscles which move the spine, or afford additional 

 security to the articulations between the vertebral pieces. Those ver- 

 tebrae which defend the posterior portions of the nervous axis, usually 

 called the spinal cord, 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, 

 are by the human anatomist distinguished as the cranium or skull. 



(1622.) 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 vertebrae, 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, how- 

 ever, waste the time of the student by considering in this place the as 

 yet unsettled and vague opinions of transcendental anatomists upon this 

 subject ; it is sufficient for our purpose merely to indicate the facial 

 bones as appendages to the cranial vertebras, avoiding for the present 

 further discussion concerning them. 



(1623.) 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. 



(1624.) The first set of arches is appended to the lateral portions of 

 the cranial vertebrae, and the bones thus derived enter largely into the 

 composition of the respiratory apparatus. In Man this important por- 

 tion of the skeleton is reduced to a mere rudiment, distinguished by the 

 name of the os hyoides ; and in the human subject its relations and con- 

 nexions with th'e surrounding parts are so obscurely visible, that the 

 student is scarcely prepared to witness the magnitude and importance of 

 the hyoid framework in other classes, or the amazing metamorphoses 

 which, as we shall afterwards see, it undergoes. 



