648 MANUAL OF PHYSIOLOGY. 



surface are in more immediate connection with certain peripheral 

 organs than are others, and that these local areas have been in 

 the habit of receiving (in the case of the visual impulses coming 

 to the angular gyri) or sending out (in the case of motor impulses 

 starting from the motor centres) impulses of a special and definite 

 kind. 



From the other facts mentioned viz., the recovery of function 

 after injury, or the complete absence of functional lesion we 

 must conclude that these local areas are by no means the only 

 agents which can carry on the business of receiving for the mind 

 impulses from the periphery, and sending out voluntary impulses 

 to the muscles ; but that rather there are many groups of nerve- 

 cells, in relation with the nearest sub-agents the basal ganglia 

 which can take on the duty of the injured cells, and act as cortical 

 centres, receiving sensory, and discharging motor impulses. In 

 respect of this capability of one part of the cerebral cortex to carry 

 on the duties ordinarily allocated to another, we have a complete 

 analogy in the gray part of the spinal cord. Partial section of 

 the gray part of the spinal cord (even if it be cut at two or three 

 different levels) does not destroy the sensation of any local area 

 of skin, showing that the delicate felt-work of nerve fibrils in the 

 gray substance can conduct the impulses in many directions, so 

 that even when a considerable number of the ordinary routes are 

 blocked by section of fibrils and destruction of the cells at the 

 part cut, the neighboring channels can carry on the work, so that 

 after a little time the sensory impulses are carried from all parts 

 of the skin to the brain without delay. 



It has already been pointed out that the function of any given 

 nerve fibre depends on the function of its terminals. The fibre 

 itself is merely a conducting agent. In somewhat the same way 

 the functions of any given nerve-cell must depend on the number 

 and character of its connections. If it be attached to a motorial 

 end-plate in a muscle, it can only be an exciter of impulses that 

 give rise to motion : if it be connected only with a sensory termi- 

 nal, it can only be a receiver of sensory impulses. But, in the 

 gray matter of the spinal cord, and still more so in that of the 

 cerebral cortex, we may assume that all the cells are in more or 



