I 88 THE GROWTH OF THK BRAIN. 



nearly all of them into the ventral columns. Finally 

 the mass of pyramidal fibres in the two halves of the 

 cord is in many cases very unequal. No better example 

 of variability in construction can be given, and from 

 this arrangement it is evident that if there is an original 

 bilateral symmetry of the central system it can be 

 modified by the subsequent growth processes. 



Besides the division of the nervous system into two 

 symmetrical halves there is another manner of dividing 

 it, which is of much morphological importance. Com- 

 parative anatomy suggests that it is formed by a series of 

 segments or equivalent portions arranged in linear series. 

 In some lower vertebrates this character is quite appa- 

 rent, and opposite to the places at which the spinal 

 nerves are attached the cord is somewhat enlarged 

 owing to the larger number of fibres which enter the 

 cord at these points, and also to the increase in the cell- 

 bodies, both in the number and in the size. 



In various ways this primitive arrangement is modified. 

 Among higher vertebrates the principal sense organs 

 located exclusively in the head assume a greater rela- 

 tive importance, and the reactions of the entire organism 

 become more and more subject to them. This depends 

 upon the fact that the various centres distributed 

 through the spinal cord become connected with the 

 cells lying at the head end in such a manner as to be 

 somewhat controlled by them. These connections are 

 mediated by bundles of fibres, which, traversing as they 

 do the length of the cord, disturb the segmentat 

 arrangement. Moreover the great development of nerve 

 elements in the cord at the regions where the nerves 

 controlling the limbs are given off, causes a very con- 

 siderable enlargement, extending through a number of 

 primitive segments. As a result of all these modifica- 

 tions, the primitive segmental character of the medul- 

 lary tube is much obscured in man. 



