HERRICK'S AN INTRODUCTION TO NEUROLOGY 431 



sure it might be represented by a number of terminal filaments, 

 by way of a ganglion to its center, there to mediate one kind of 

 change the specific central response for that fiber. The struc- 

 tural picture on which this doctrine was based can no longer be 

 accepted. We now know that the afferent neuron sends into 

 the central system an axon which always branches and often 

 forms connections remote from one another and plainly associated 

 with diverse correlation centers, while the nerve fiber passing to 

 the periphery from this same ganglion cell in many cases branches 

 several times in its course, sends these branches to different 

 peripheral localities and at the periphery may, in the skin at 

 least, be connected with more than one type of sensory end 

 organ. This calls for a reconsideration of the problem and it 

 seems possible, therefore, to think of nerve impulses as having 

 distinctive characters that determine which one of the many 

 possible paths each type of impulse is to follow within the central 

 system. Here is a field open for further study. In man the 

 pathway for pain formed by central neurons in the spinal cord 

 is represented both by a diffuse arrangement as in lower forms 

 and also by a well marked tract. (Tractus spino-thalamicus 

 lateralis). The recent observations of Head and Holmes make 

 it probable that in the cell groups of the thalamus, where these 

 fibers end, we have a center stimulation of which gives rise to 

 our sensations of pain. It has been customary to associate 

 changes in consciousness with the activity of the cortex only, 

 but in view of these results we must now grant certain cell groups 

 in the thalamus a like dignity. 



The last chapters of the book treat of pleasure and pain (we 

 need a better word antithetic to pleasure) and of the human 

 cortex; its structure, functions and evolution. In the course of 

 these chapters a new picture is given of the relation of the higher 

 centers which increase in complexity from the bulb to the thala- 

 mus and from the thalamus to the cortex. In place of the usual 

 notion of the impulses from complex higher centers acting on 

 lower centers which are more or less indifferent or passive, our 

 author points out that the incoming impulses set or adjust for 

 discharge circuits of increasing complexity, as these impulses 

 pass to higher and higher centers and as a consequence the re- 

 turn impulses from the cortex which release the final response, 

 act by selecting one out of several circuits which are held ready 



