THEORY OF THE SKULL. 461 



fusing with the ossified remainder of the cartilaginous 

 cranium, form a complete bony capsule. In Cyclostomes 

 and Elasmobranchs the brain-box is wholly cartilaginous ; 

 above Elasmobranchs the cartilage is more or less thoroughly 

 replaced or covered by bones. In the individual develop- 

 ment there is a parallel progress. 



Theory of the skull. Near the beginning of the nineteenth 

 century, Oken and Goethe independently propounded what is known as 

 the vertebral theory of the skull. Regarding the skull as an anterior 

 portion of the vertebral column, composed of three or four vertebra?, they 

 compared the bones of the different regions to the parts of a vertebra. 

 Thus in the hindmost region of the skull, the basi-occipital, the two ex- 

 occipitals, and the supra-occipital were held to correspond to the centrum, 

 the neural arches, and the neural spine of a vertebral body. 



This undoubtedly suggestive theory, modified in various details, per- 

 sisted for a long period, but ultimately gave way before the advances in 

 comparative anatomy and embryology. Huxley gave it its deathblow, 

 and Gegenbaur replaced it by what may be called the segmental theory 

 of the skull. 



To realise this theory we must go back in development to the period 

 before the mesoblast has ensheathed the notochord. At this time the 

 segmentation of the body is expressed, not in the skeleton (notochord), 

 but in the primitive segments. The segments, though less obvious 

 than in the trunk, are represented in the head region. Formerly nine 

 were enumerated, but it appears that in Elasmobranchs they are 

 more numerous. Subsequently brain and spinal cord become alike 

 enveloped in the mesoblastic sheath, which gives rise to the skeleton of 

 both head and trunk. 



The great development of the muscle segments of the trunk region 

 induces a secondary segmentation of the mesoblastic skeleton (vertebral 

 column), while the slight development of the muscles of the head region 

 exercises no such influence upon its skeleton ; this is therefore always 

 quite devoid of segmentation. The segmentation of the head, in con- 

 tradistinction to the skull, is expressed, although indistinctly, by the 

 muscle segments and by the nerves supplying these, perhaps also by 

 the lateral sense organs, the ganglia, and the arches. While it is quite 

 certain that it is the head that is segmented and not the skull, the 

 details of the segmentation are still much debated. 



Vertebral column. A dorsal skeletal axis is character- 

 istic of Vertebrata, and its usefulness is evident. It gives 

 coherent strength to the body ; it is usually associated very 

 closely with a skull, with limb girdles, and with ribs ; it 

 affords stable insertion to muscles ; its dorsal parts usually 

 form a protective arch around the spinal cord. 



To understand this skeletal axis, we must distinguish 

 clearly between the notochord and the backbone. 



