356 Mr. H. G. Seeley on a Theory 



distiuguish the more anterior or sensory part of the neural column 

 and canal from the part which is always more or less uniform, 

 and is called the spinal column ; it also exhibits the fact that 

 a mouth may exist without having the least connexion with the 

 cranium, — thus showing that just as a skull must be a result of 

 functional development of the organs of sense at one end of the 

 nervous column, so by modification the apparatus around the 

 commencement of the digestive canal takes the form of jaws and 

 facial bones. Thus, however close the jaws may be brought in 

 contact with the cranium, and however the primitive cartilages 

 which form the prehensile end of the digestive canal may be 

 modified by adaptation to other ossifications, they constitute a 

 structure which can only owe its development, like everything 

 else, to the higher requirement, or differentiation, of the func- 

 tion in which it took its rise ; and so, though forming no part 

 of the original structure of the cranium in the lowest vertebrata, 

 it constitutes by adaptation in higher forms of life an essential 

 part of the skull. And, on the other hand, since the cranium 

 is sometimes wanting (and in Amphioxus there is nothing which 

 can be separated from the spinal cord as a brain), it would be hard 

 to regard any brain as more than a functional overgrowth of the 

 end of the spinal cord, and therefore to do otherwise than believe 

 that its osseous case would be originally formed on the same plan 

 with the vertebrae, yet speedily and enormously modified by the 

 different functions which it subserves. Then, just as the brain, 

 from being inseparable from the spinal cord at first, comes 

 at last to be a structure as distinct as may be, there is here a 

 modification not unlike that which separates the segments of a 

 limb (only greater), so that, though both are parts of the same 

 organ, their structure and functions are very difi'erent. And there- 

 fore, although the covering of the brain may in some organisms 

 be inseparable from the vertebrae, there can only be expected to 

 be the same degree of correspondence between the skull and the 

 vertebral column that tliere is between the brain and the spinal 

 cord. If a brain has parts which have no representatives in the 

 spinal cord, it will not be surprising if the brain-case has parts 

 which arc not found in the case for the spinal cord. 



If a skull is examined, it will be found to be the outlet for, or 

 rather the entrance to, the nervous system ; this part is occupied 

 by the brain. Secondly, it is the entrance to the digestive sys- 

 tem ; and this part is constituted by the jaws. And, lastly, it is 

 the entrance of the lungs, respiration being carried on through 

 the nasal apertures. All these several forces of eating, breathing, 

 and observing and thinking exercise great pressure and tension 

 on the regions they affect ; and it is precisely these which we have 

 already seen ossifying the skeleton. Seeing how the small epi- 



