Manchester Memoirs, Vol. xlii. (1898), No. \%, 33 



Trace out the varied chain, and whether you follow the 

 short loops or the long ones you will come back to a 

 muscle or some analogous effective unit. Run along the 

 optic fibre and tract to the corpus quadrigeminum, a short 

 loop brings you back to ocular and other muscles (there 

 may be a still shorter loop within the retina itself). Pass 

 upward to the occipital cortex, a longer loop brings you 

 back once more. Pass on from the occipital cortex along 

 the association fibres first to one and then to another part 

 of the cortex, loops, longer it is true, but still loops bring 

 you back to muscle. During all this, at each link of the 

 chain, the differentiation pertaining to each unit brings 

 about a change in the nature of the events taking place. 

 At the beginning we call the events simply neural. Later 

 on we recognize them as psychical. As we have seen, we 

 seem to have clear reason to conclude that in the occipital 

 units the threshold of the psychical has been reached, 

 but we have also seen reasons to think that it has not 

 been reached at one jump, but rather by successive 

 differentiations. We may infer that in the so called 

 " associations " loops, further successive psychical differen- 

 tiation is effected. And, indeed, positive evidence is being 

 gathered that this is the case. 



I must not enter fully into this part of my subject, 

 since I have other points to dwell on ; I must content 

 myself with one example. 



One of the muscular mechanisms with which the visual, 

 nervous chain is connected is that of speech. When a 

 person in reading speaks the word which he sees before him, 

 a nervous machinery of many links intervenes between 

 the image of the word thrown on the retina, and the mus- 

 cular contractions which give rise to the sound of the 

 spoken word. That machinery may be damaged by injury 

 or disease in many ways, the chain may be broken in one or 

 other of the many links. It may be broken in the following 



