16 



NATURAL HIS TOR y OF VERTEBRATES. 



Fig. 18. — Heail of sturgeon, 

 sliowing tlie membrane bones 

 covering the (dotted) cbondro- 



of the head ; they likewise occur in the cavity of the mouth, where the teeth more 

 usually persist than on the free surface of the head. It is to be noted that, just as in 

 the case of the scales of many fishes, the cement plates may remain, although the teeth 

 (their raison d^etre) have disappeared, so that cement bones 

 are found on the skull and in the cavity of the mouth, with- 

 out a trace of teeth. 



Such cement bones are also spoken of as dermal bones, 

 or investing bones, from their relation to the skin, or to the 

 underlying skidl respectively ; but it is also Usual to speak of 

 another category of bones as dermal, Aviiich have an entirely 

 different origin from tlie above, and which, indeed, are de- 

 veloped for the protection of the neuromastic canals referred 

 to before. Such neuromastic bones are not confined to the 

 aquatic vertebrates, but retain their relationship to the skull 

 in air-breathing forms, although their original function has 

 disap]:ieared. 



The cement bones and neuromastic bones are something 

 superadded to the cartilaginous skidl, and we shall find that 

 they are not confined to the skull proper, but extend to those 

 modified visceral arches which, roughly speaking, give rise to 

 the jaws. But certain bones originate in closer i-elntionship 

 to the cartilage ; they may first ajijjear in the centre of cer- 

 tain cartilaginous tracts, and gradually replace the cartilage 

 centrifugallj', or they may appear in the membrane which 

 clothes the cartilage (the perichondrium), and, eating into it, as it were, replace it 

 centripetally. These are generally s]>oken of as ' cartilage bones,' as distinguished from 

 the 'membrane' bones of the foregoing paragrajih ; but it is impossible to draw a very 

 sharp distinction between them, for a bone which originates round a neuromastic canal, 

 for e.xample, may eat likewise centripetally into the underlying cartilage. 



Again it may be impossible, e.Yce]>t in the cavity of the mouth, to distinguish 

 between the limits of neuromastic and cement bones; for two bones originating in 

 these two different manners may fuse. It will therefore be sufficient to indicate the 

 thief places where the chondrocranium and the cartilages of the jaws are replaced by 

 the 'cartilage' bones, and where they are invested on the outside by 'membrane' 

 bones. 



It has been remarked above that a demarcation of the skull into regions is effected 

 by the sense organs, but these regions are not 'segments.' So also the points of escape 

 of the cranial nerves give valuable landmarks in the study of the skull, but neither do 

 these mark ofT segments ; for although many of them, as we shall see, are comjiarable 

 to spinal nerves, yet they have lost the primitive segmental arrangement which the 

 spinal nerves have retahied. 



In discussing the bony framework of the skull, it will be convenient to take the 

 brain-box first, and the jaws afterwards. The occipital region generally exhibits four 

 bones surrounding the foramen magnum or aperture through which the brain commu- 

 nicates with the spinal cord. These are the basioccipital below, the sujiraoccipital 

 above, and the two exoccipitals, one at either side (Fig. 19), but the latter may alone 

 appear in this region, as in the frog, the rest of the region remaining cartilaginous. 

 The bones of the auditory region are more numerous ; their limits are gener.ally detectable 



