VERTEBRATA. 483 



the vertebrae consist of the body (a), the superior arch (b) and 

 spinous process (c), and the inferior arch ( e) and spinous pro- 

 cess (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, therefore, that 

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

 simple arrest of the developement of certain elements, and the 

 disproportionate expansion of others, until at length it becomes 

 scarcely recognisable as constituting the same piece of the ske- 

 leton. 



Who would be prepared to expect, for example, that the occi- 

 pital 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 pro- 

 portionate length of the transverse processes (W), we arrive at 

 the form of a human vertebra, which exhibits precisely similar 

 elements : enlarge the arches (6, b) that surround 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 imper- 

 ceptibly at the occipital bone of the human cranium ; the main 

 differences 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. 



One other illustration of this interesting subject. What bones 

 compose a completely formed thorax ? In man we find, as every 

 tyro knows, 1st. the dorsal vertebra; 2dly. the ribs, with their 

 cartilages ; and 3dly. the sternum. But it is not in man that we 

 must expect a perfectly developed thoracic framework ; it is in the 

 birds that are destined to rise in the air by the assistance of their 

 proportionately powerful thoracic extremities. If therefore we 

 examine the thorax of a bird, we find it composed of pieces which 

 in man are absolutely wanting: we see 1st. the vertebra; #dly. 

 the dorsal ribs, firmly articulated on each side both with their 

 bodies and transverse processes ; 3dly. the sternal ribs, extending 

 from the ribs last mentioned to the sternum ; and, lastly, the 

 sternum, itself composed, as we shall afterwards see, of various 



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