THE SKULL OF OSSEOUS FISHES. 89 



c. The bones of the dermo-skeletoii are : — 



Nos. of bones. 



Supra-temporuls - - - 71 



Supra-orbitals - - - - 72 



Sub-orbitals ... 73^ 73' 



Labials - - - - - 74* 



SUPERIOR (neural) AKCIIES OF THE CRANIAL VERTEBRAE. 



The first series of endo-skeletal bones constitutes the axis or back- 

 bone of the skull, as the rest of the vertebral neural arches do that 

 of the trunk ; and it includes and protects the encephalou or anterior 

 expanded extremity of the great nervous axis. The under and 

 upper parts of the annular segments are commonly formed by single 

 and symmetrical bones, as in the vertebral axis of the trunk ; but 

 sometimes, even in the present low class, the expansion of the cranial 

 cavity is accompanied, not only by a transverse development, but also 

 by a median division of the upper piece or key-bone of one or more 

 of the protecting arches. 



Though subject to various degrees of anchylosis, the cranial ver- 

 tebra? always accord in number with the primary ganglions or divi- 

 sions of the encephalon in Fishes. For the better understanding of 

 this important relation, I may premise that the brain of Fishes con- 

 sists of four primary divisions succeeding each other in a linear 

 series horizontally, which, viewed from behind forwards, are : — 



1. The medulla oblongata, with the superimposed cerebellum, or 

 the ' epencephalon.' 



2. The third ventricle, with its upper (pineal) and lower (hyjjo- 

 physial) prolongations, and the superimposed optic lobes, or the 

 ' mesencephalon.' 



3. The cerebrum proper, or ' prosencephalon.' 



4. The olfactory ganglionic or chord-like prolongation of the 

 cerebrum, or ' rhinencephalon.' 



In most osseous fishes, as in this disarticulated skull of the Cod 

 ( Gadus Morrhua), the bones encompassing, or in vertebral relation 

 with, the eiicncephalon are six in number (Jit/. 30. and Jit/. 31. i.). 



* In the human skull the only bones that can, with any probability, be referred 

 to the ilennal system are the ' lachrymal.' The splanchnic system is reduced to 

 the capsules of the organs of sense, of which only those of the ear and nose are 

 ossified. The endo-skeletal bones form the same number of neural and liwmal 

 arches as in the fish, but that of the occipital vertebra is far removed from its cen- 

 trum, and neither the mandibular nor hyoidean arches retain diverging appendages. 



