THE SKULL 65 



which the skull consisted of a number of modified vertebrae 

 (" cranial vertebrae "). On this theory, therefore, the skull Avas 

 regarded as a special modification of the anterior part of the 

 vertebral column, and a large number of facts were brought 

 forward in support of it : even when morphological science had 

 made further considerable advances, there still seemed to be a 

 certain amount of justification for it. 



The arguments in support of the vertebral theory of the skull 

 may be briefly stated as follows. As in the vertebral column, three 

 stages may be distinguished in the skull, ontogenetically as well as 

 phylogenetically: viz., a membranous, a cartilaginous, and a bony stage 

 (comp. p. 36). There is thus an important correspondence between 

 these two parts of the cranio- spinal axis, and this is further em- 

 phasized by the fact that the notochord always extends for a certain 

 distance into the base of the skull, so that the latter is developed on 

 the same skeletogenous basis as. and in direct continuation of, 

 the vertebral axis. 



This theory depended on giving an exact account merely of the 

 skeletogenous elements taking part in the formation of the skull, 

 and for a long time it was not recognised that this could not 

 possibly lead to a true interpretation of the origin of the verte- 

 brate head. To attempt to do so was to " put the cart before the 

 horse," by looking upon the last acquisition of the head its skeleton 

 as the leading point for future researches. 



It was only very gradually ascertained that the skull has never 

 consisted of segmentally arranged cartilaginous portions, either in 

 the course of its ancestral history or in that of the development of 

 the individual. In the occipital region alone did it possibly at one 

 time possess distinct neural arches, owing to the assimilation of 

 more or fewer of the anterior segments of the tiunk ; and the 

 view gradually gained ground that this important problem could 

 not be solved merely by an anatomical and embryological analysis 

 of the skeleton, but that a number of other parts and organs 

 which arise much earlier must also be taken into account 

 and their origin traced : such are, the sensory organs, brain and 

 cerebral nerves, cranial muscles, and the anterior part of the 

 alimentary canal together with the mouth and visceral clefts. 



A. considerable advance was thus made, and the problem was 

 vigorously attacked both from the anatomical and embryological 

 sides ; and many of the researches which resulted have become 

 classical in the history of the subject. It is impossible here to 

 give more than the barest outlines of the results obtained, and 

 even now much remains to be elucidated in this complex question, 

 about many details of which numerous differences of opinion still 

 exist. Moreover, a knowledge of the development and distribu- 

 tion of the cerebral nerves is a necessary preliminary to the study 

 of cranial morphology : these are treated of in a subsequent 

 chapter, to which the reader is referred for explanation of parts of 

 the following paragraphs. 



