446 COMPAEATIVE Als^ATOMY. 



§ 340. 



The relations between the cephalic skeleton and the vertehral 

 column were, in past times, the cause of attempts to show that the 

 former was made up of various segments resembling vertebra? ; from 

 this point of view the cephalic skeleton seemed to be merely a modi- 

 fication of the vertebral column. It was indeed believed that the 

 characters of the various segments of the osseous cranium would 

 be of aid in this comparison ; as a fact, however, these are of very 

 uncertain value, as in dealing with them we have to do with a 

 condition which has already been much modified. In other words, 

 the cephalic bones which have been said to belong to three, four, or 

 five so-called " cranial vertebrae," have had very different origins, and 

 are for the most part structures which primitively did not belong to 

 the cranium. 



The examination of the primordial cranium of the lower Verte- 

 brata, especially with reference to the nerves which pass out from it, 

 shows that, although indications of its primitive composition out of 

 metameres, homodynamous with vertebrte, can be made out, as 

 might be supposed indeed, yet this metamerism of the cranium is not 

 the same as that segmentation which is faintly indicated by the bony 

 cranium. 



The view here adopted is chiefly based on the following points : 



1) It can be shown that the arches of the branchial skeleton 

 form inferior arches which belong to the cranium. 



2) It follows that a general agreement may be made out between 

 the visceral arches and the inferior arches of the vertebral column. 



3) The cranium is comparable to a portion of the vertebral 

 column, which contains, at least, as many vertebral segments as there 

 are branchial arches. 



4) In the cranium itself there are a number of important points 

 in which it resembles the vertebral column. 



a) The chorda dorsalis which forms the base-work of the 

 vertebral column passes through the cranium in just the 

 same way as it passes through the vertebral column. 



I) All the nerves which pass out in this segment are homo- 

 dynamous with the spinal nerves. 



c) The differences seen in the cranium, as compared with the 

 vertebral column, may be explained as adaptations to certain 

 conditions, which are external to the cranium; that is, 

 as acquired arrangements. They lead us, therefore, to 

 presuppose a condition, in which the cranium had not yet 

 acquired these peculiarities, and when therefore it did not 

 differ very greatly from the vertebral column. 



5) The differentiation of the cranium appears therefore to 

 be due to the concrescence of a number of vertebrae : such 



