278 THE MECHANICAL FUNCTIONS. 



the latest of those repetitions, becomes less perfect, and ends 

 by being abortive. In the present instance, the consequences 

 of this law are highly advantageous, since it provides for 

 the flexibility of the tail, and qualifies it for being applied 

 to a great variety of useful purposes, as we find more espe- 

 cially exemplified in the Ateles, or spider monkey, and in 

 the Kangiiroo, 



Next in importance to the spine is the craniuvi, or osse- 

 ous covering of the brain; together with the bones of the 

 face, which protect the organs of the finer senses. An ac- 

 curate investigation of the mode in which these bones are 

 formed has led many modern anatomists to the opinion 

 that they were originally parts of the spinal cokimn, and 

 that they are, in fact, developments of vertebrae, much al- 

 tered, indeed, in shape, in consequence of the new condi- 

 tions to which they have been subjected; but still possessing 

 all the essential elements of vertebrae. In the embryo con- 

 dition of these organs, and while the brain is yet undeve- 

 loped, the resemblance of the bony circles which enclose it 

 to vertebrae is certainly very striking; but in proportion as 

 the brain becomes expanded, the similarity diminishes; for 

 the rapid growth of the brain in the higher orders of animals 

 is necessarily attended with an equally sudden expansion of 

 the bones of the skull. Hence, their several elements are 

 thrown into unusual positions, and being variously distorted 

 and disfigured, can hardly be recognised under the strange 

 disguises they assume. 



The extensive researches that have been recently made in 

 this branch of comparative anatomy, have supplied many 

 facts, which tend to support the hypothesis that the bony 

 coverings of the brain are the result of the development of 

 three vertebrae. According to this theory, the first of these 

 supposed cranial vertebrse, beginning our enumeration from 

 the neck, is the origin of the occipital bone, of which the 

 lower part, or that which immediately supports the cerebel- 

 lum, corresponds to the body of the vertebra; the two lateral 

 portions to the leaves; and the upper flat plate, to the spinous 

 process. The body of the second cranial vertebra becomes, 



