40 DB. HILL, FROM REFLEX ACTION TO VOLITION. 



In vertebrate animals, a second field of plexus, the cortex 

 of* the brain, is superadded to the primitive grey matter of the 

 spinal cord. Several questions must however be answered 

 before we can correlate this with any distinct advance in the 

 performance of the nervous apparatus. We do not know 

 whether the elements contained in this cortical formation 

 arc already present in invertebrate animals; nor do Ave know 

 what relation the great masses at the base of the cerebrum, 

 the corpora striata (which constitute the bulk of the birds' 

 brain), bear to the mantle or cortex proper, which does not 

 really make its appearance amongst vertebrates until the 

 cartilaginous fishes are reached. Much as we should like to 

 regard the first appearance of the cortex as a great step in 

 advance in the evolution of the central nervous system, we 

 must at present refrain from drawing wide conclusions, and 

 merely note the fact that the nervous system of higher 

 animals is divisible into two parts ; that it consists of a vast 

 aggregation of elements which, so far as we can see, are not 

 marked by any differences in character or even in arrange- 

 ment. The libres which eonncet the inner grey matter with 

 the surface of the body, pour their impulses into a network 

 from which motor fibres take origin. The grey matter 

 presents therefore a variety of routes through which sensory 

 impulses may flow over into motor paths, each sensory-motor 

 path ('(instituting an are. 



But on the ares which collectivelymake up the lower system 

 are superadded ares, the loops of which lie in the higher grey 

 matter. At the same time, therefore, that an impulse flows 

 across the spinal cord, as a simple direct reflex action, a certain 

 part of this impulse is also diverted to the brain along fibres 

 which ascend in the outer part of the spinal cord; and from 

 the brain descending fibres carry the impulse back again to 

 the lower arc. Let us try to avoid expressions which convey 

 a more concrete idea of the relations of these two sets of arcs to 

 one another than we are at present justified in formulating; 

 and above all let us avoid terms derived from commerce, 

 military organizations, and other developments of social 

 activity which would be but coarse symbols to apply to this 

 relation, of the exact nature of which we are able to form 

 but a dim picture, and let us say that we see in the super- 

 posed arcs of which there may be, for aught wo know, not 

 two only, but many, opportunities for the control, the rein- 

 forcement, the restraint of reflex action. Accurate measure- 

 ments of the time taken by impulses in travelling through 



