15^ THE GROWTH OK THK BRAIN. 



In this latter process the medullary sheath plays an 

 important role. That there are axis-cylinders which do 

 not have this sheath is probable, but its presence in con- 

 nection with so many neurons makes it an important 

 structural feature, and whatever view may be taken of 

 its origin does not alter the fact that it is the companion 

 of the axis-cylinder in all parts of the central system. 

 The larger number of dendrons possessed by the more 

 specialised cells suggests that in these cases there has 

 been an increase in the number of channels through 

 which the cell-body can receive stimuli, and by conse- 

 quence its reactions become subject to more numerous 

 modifications. 



Thus far in this description the cell-body and the 

 neuron have alone been described, while the dendrons 

 have been neglected. However these latter are inter- 

 preted, whether as pathways or afferent impulses, or as 

 merely nutritive channels, their importance is great, and 

 variations in their number and size must be of profound 

 significance, despite the fact that their total volume is 

 very small. As will be seen by referring to the figures 

 taken from Cajal (Fig. 21), the dendrons develop later 

 than the neuron, and have also a different morphological 

 character. They tend therefore to be more abundant in 

 the older cells of the same individual, and in those taken 

 from the animals high in the zoological scale. 



In addition to the size of the nerve elements, it is also 

 desirable to determine their number, either in the entire 

 nervous system or in definite portions of it. If we knew 

 the volume of the nervous system at any age, and at the 

 same time the average volume of the nerve elements, 

 the determination of number would be a simple matter 

 of arithmetic. Unfortunately much uncertainty attaches 

 to the data which must at present be employed, and the 

 results -are correspondingly unsatisfactory. Tabulated 

 below are some figures which will be of assistance. 



