ELEMENTS OF THE VERTEBRAE. 611 



(1632.) The question, therefore, to be solved is this how many 

 elements exist in the most perfect vertebra known ? and this being once 

 satisfactorily settled, it is easy to detect the deficiencies of such as are 

 less completely developed. 



(1633.) Taking the example above given as a specimen of a fully- 

 formed vertebra, it has been found to be divisible into the following 

 pieces, all or only a part of which may be present in other vertebrae 

 even belonging to the same skeleton ; and these parts are represented 

 detached from each other in the diagram which accompanies the figure 

 (fig. 306, B). They are, 1st, the centre or body of the bone ; 2ndly, two 

 elements (b b), which embrace the spinal marrow ; 3rdly, the superior 

 process (c) ; 4thly, the two transverse processes (d) ; 5thly, two ele- 

 ments forming the inferior arch, and enclosing the principal blood-vessels 

 (e) ; and 6thly, an inferior spinous process (g\ 



(1634.) With this key before us, we are able with the utmost ease to 

 comprehend the structure of any form of vertebra that may offer itself. 

 Thus, in different regions of the back of the same Fish, the composition 

 of the vertebra is totally different : near the tail the vertebrae consist of 

 the body (a), the superior arch (b) and spinous process (c), and the in- 

 ferior arch (e) and spinous process (g). In the neighbourhood of the 

 head, however, neither the inferior arch nor spinous process are at all 

 developed; but the transverse processes, which were deficient in the 

 former case, are here of great size and strength. It is obvious, there- 

 fore, that the form of a vertebra may be modified to any extent by the 

 simple arrest of the development of certain elements and the dispropor- 

 tionate expansion of others, until at length it becomes scarcely recog- 

 nizable as constituting the same piece of the skeleton. 



(1635.) Who would be prepared to expect, for example, that the 

 occipital bone of the human head was merely a modification of a few of 

 the elements of the Fish's vertebra above described enormously expanded, 

 in order to become adapted to altered circumstances ? And yet, how 

 simple is the transition ! By removing the inferior arch (e) and spinous 

 process (g), and slightly reducing the proportionate length of the trans- 

 verse processes (d), we arrive at the form of a human vertebra, which 

 exhibits precisely similar elements. Enlarge the arches (b b) that sur- 

 round the spinal axis of the nervous system, increase the size of the 

 superior spinous element (c), and we have the occipital bone of a Fish ; 

 and from hence, through a few intermediate links, we arrive almost im- 

 perceptibly at the occipital bone of the human cranium, the main dif- 

 ferences being that the body is in Man divided into two lateral halves, 

 while the superior arches (b) become spread out so as adequately to 

 defend the prodigiously-developed masses of the brain, to which in the 

 human body they correspond. 



(1636.) One other illustration of this interesting subject. What 

 bones compose a completely-formed thorax ? In Man we find, as every 



