o6 



ELEMENTA K Y ANA TO MY. 



[LESS. 



membranous investment of this terminal part of the spinaJ 

 marrow. 



In other Vertebrates a definite and distinct brain, en- 

 closed in a definite and distinct brain-case of solid tissue 

 (cartilaginous or osseous), with jaws and hyoidean appendages, 

 is almost a constant character. 



In that the skull is sharply and distinctly differentiated off 

 from the spinal skeleton, man agrees with the vast majority 

 of Vertebrates, and with all Vertebrates without exception 

 above Fishes. It is possible, however, for the cartilaginous 

 representatives of vertebrae to coalesce into one mass with 

 the cartilaginous skull (as in the Sturgeon), or even when the 

 skeleton is osseous (as in the Siluroid fish Bagrns, where 

 these bones are suturally united). Indeed, in such cases, the 

 transition from spine to cranium is so gradual that it is easy 

 to mistake part of the vertebral column for part of the skull. 



FIG. 94. VERTICAL, LONGITUDINAL SECTION OF THE POST-AXIAL PART OF THE 

 SKULL, AND OF THE MORE PRE AXIAL VERTEBRA OF THE SILUROID FISH 

 Bagnis. (After OIVCH.) 



at, body of first vertebra ; ax, body of second vertebra ; -; 3 z> 7 , bodies of the 

 next five vertebrae, each line going to the middle of a biconcave centrum ; bo, 

 basilar part of the occipital bone ; co, exoccipital ; 5, spinous process of the 

 second vertebra. Directly above ^ is the post-axially projecting supra-occi- 

 pital. Hy, the hypapophysial canal running beneath the first five vertebrae. 



The almost completely osseous condition of the skull of 

 man is one common to him and to his class. In the Sauro- 

 psida the skull is often eked out, as it were, by considerable 

 tracts of cartilage or membrane, and in the Ichthyopsida 

 the cartilaginous portion is always more considerable, and 

 may constitute the greater part or the actual whole of the 

 solid brain-case and annexed structures. 



The shape of the skull will be spoken of afterwards, but 



