124 TROPHIC NERVE INFLUENCE. 



On the other hand, we must also remember that there must 

 be a specific apparatus at the periphery of sensific nerves 

 which shall enable them to take cognizance of, and express as 

 a stimulus in vis nervosa, the different forms of pressure-force 

 into which the objective cause of. the special senses, except 

 taste and smell, may be resolved. Helmholtz has revived the 

 theory of Young, that distinct peripheral nerve apparatus in 

 the retina take cognizance of the cardinal elements of colour ; 

 and has extended the same theory to the sense of hearing, in 

 which he supposes that a separate nerve fibre takes cognizance 

 of each definite note to which the rods of Corti vibrate in con- 

 sonance. The structure of the retina in birds and reptiles, as 

 described by Max Schultze, and the extreme fineness of the 

 ultimate peripheral nerve fibrils, as described by Beale, give an 

 anatomical basis for the hypothesis. Thus the nerve force, as 

 a stimulus, may be very similar everywhere, and may be a mere 

 sign, whose specificity lies in the interpretation given at the 

 nerve centre where it acts ; and also in the speciality of the 

 peripheral apparatus, enabling the living matter to take cogni- 

 zance at all of the different forms of external force as 

 stimuli. Nevertheless, we can hardly suppose the existence of 

 separate nerve fibres for all the numerous varieties of sensa- 

 tions depending on stimulation through nerve fibres ; so we 

 must conclude there are some qualitative differences in the vis 

 nervosa capable of being conveyed through one and the same 

 fibre. 



Beale would apparently reduce the whole of efferent 

 nerve action to causing a simple plus or minus 

 of muscular contraction, voluntary and involuntary, 

 and the greater part of the afferent nerve action to 

 furnishing the stimulus for the latter; for he denies 

 all direct nerve action on the living matter in secretion 

 and nutrition, any such action being merely indirect, 

 through the change of calibre of the capillary arteries 

 from muscular action. His theory is here surrounded 



