292 



THE EVOLUTION OF MAN. 



the foremost, peculiarly modified section of the vertebral 

 column, develops in an exactly similar manner. Just as, 

 in the spinal column, the vertebral canal envelopes and pro- 

 tects the dorsal marrow, so the skull forms a bony covering 

 round the brain ; and, as the brain is merely the anterior, 

 peculiarly differentiated portion of the dorsal marrow, we 

 might conclude on a priori grounds, that the bony envelope 

 of the brain is a peculiar modification of that of the dorsal 

 marrow. It is true, that if the developed human skull 

 (Fig. 264) is considered by itself, it is impossible to under- 

 stand how it can be merely the modified anterior portion of 

 the vertebral column. It is a complex, capacious bony 

 structure, consisting of no less than twenty bones, differing 



widely in form and size. Seven of 

 these skull-bones constitute the 

 spacious case which encloses the 

 brain, and in which we distinguish 

 the strong, massive floor of the skull 

 (basis cranii) below, and the 

 boldly arched roof of the skull 

 (fornix cranii) above. The other 

 thirteen bones form the "facial 

 skull," which especially provides the bony envelopes of 

 the higher sense-organs, and at the same time as the jaw- 

 skeleton, encircles the entrance to the intestinal canal. 

 The lower jaw (usually regarded as the twenty-first 

 skull-bone) is jointed to the skull-floor, and behind this, 

 embedded in the roots of the tongue, we find the tongue- 

 bone, which, like the lower jaw, has originated from the 

 gill-arches, together with a portion of the lower arch, which 

 originally developed as " skull-ribs " from the ventral side 

 of the skull-floor. 



Fig. 264.— Human skull, 

 from the right side. 



