CRANIUM. 355 



become stinted in their growth and ossification, 

 until we find the terminal pieces generally remain- 

 ing in the state of cartilage. Such is the structure 

 of the osseous support of the tail, as seen in many 

 quadrupeds in its most developed forms. It illus- 

 trates the law, that when in any system there 

 occurs a frequent repetition of the same structure, 

 the evolution, in the latest of those repetitions, 

 becomes less perfect, and finally ceases. In the 

 present instance, the consequences of this law are 

 highly advantageous, since it provides for the flexi- 

 bility of the tail, and qualifies it for being applied 

 to a great variety of useful purposes, as we find 

 more especially exemplified in the ^^e/6.s, or spider 

 monkey, and in the Kanguroo. 



Next in importance to the spine is the cranium, 

 or osseous covering of the brain ; together with the 

 bones of the face which protect the organs of the 

 finer senses. A diligent 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 column, and that they 

 are in fact developements of vertebrae ; much al- 

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

 conditions to which they have been subjected ; but 

 still possessing all the essential elements of ver- 

 tebrai. In the embryo condition of these organs, 

 and while the brain is yet undeveloped, the bony 

 circles which enclose it certainly resemble ver- 

 tebrae ; but in proportion as the brain becomes ex- 

 panded, the similarity diminishes; for the rapid 

 growth of the brain in the higher orders of animals 

 is necessarily attended with an equally sudden ex- 

 pansion of the bones of the skull. Hence their 



