LOCALISATION OF FUNCTION. 251 



down the initial segmental arrangement by conden- 

 sation, in such wise that groups of centres might act in 

 concert and at the same time any one of them be 

 brought under the combined control of several sense 

 organs. In discussing the ideas of localisation which 

 have developed around these observations, it will be 

 desirable to restrict ourselves here mainly to that side 

 of the question which will throw most light upon the 

 architecture of the central nervous structures. 



The architecture has already been examined from the 

 anatomical point of view, as well as embryologically ; 

 and it has been pointed out that probably the larger 

 divisions of the encephalon, which are of such interest 

 to us, are the homologues of that region in the spinal 

 oord which forms the dorsal horns and contains a large 

 proportion of the central cells. It remains to be 

 determined how pathways for incoming impulses are 

 there related to those for outgoing ones, and it is in this 

 direction that the physiological experiments yield facts 

 which can be utilised. 



But, in the first instance, the problem may be dealt 

 with in a purely statistical manner. Belonging to the 

 spinal cord there are thirty-one pairs of nerves by which 

 impulses enter and leave it. There are also enumerated 

 within the cranium twelve pairs of nerves, of which six 

 are predominantly pathways for incoming impulses, and 

 the other six for those outgoing. I say predominantly, 

 because, as a matter of fact, in the optic and olfactory 

 pathways there are also some fibres which transmit 

 outgoing impulses. Such being the formal arrange- 

 ment by which the central system is connected with the 

 other parts of the body, we may study a given locality 

 in it with a view to obtaining further details. 



Some idea of the central connection of the nerve 

 roots can be got from the arrangement of the motor 



