CENTRAL NERVOUS SYSTEM. 675 



last analysis the physiological condition for pain is excessive stimulation, 

 which by all analogy must mean excessive discharge within the central system. 

 The changes following this discharge into the central system are not such as 

 lead to co-ordinated muscular responses, but to convulsive reactions of a very 

 irregular character. Where this process takes place in the central system we 

 do not know, because we can only determine the existence of this sensation 

 when conscious. As to normal analgesia, it must be looked upon as 'depend- 

 ent on a condition in which excessive stimulation cannot be produced; and we 

 find this condition normally present in the case of the nerves of special sense. 



Returning now to the arrangements by which the several dermal sensations 

 are mediated, the hypothesis may be entertained that one peripheral twig 

 of a dermal nerve may be modified for thermal and another for mechanical 

 stimulation, and, though they run by way of the same ganglion-cell, may yet 

 find a different distribution in the centre, and thus lead to different sensations. 



Since in the pathological cases the one sort of sensibility may be lost while 

 the others remain, it has been inferred that there were separate fibres for the 

 conveyance of each sort of sensation. This idea was expressed in the law of 

 the specific energies of nerves as formulated by Johannes Miiller, who pointed 

 out that in many cases the same nerve might be stimulated in any way, me- 

 chanically, electrically, or chemically, as well as in the normal physiological 

 manner, and that in all cases the mode of the response was the same a sen- 

 sation of light or taste or contact, as the case might be. Hence it was argued 

 that the mode of the sensation was independent of the kind of stimulus, but 

 dependent on the nature of the central cells, among which the afferent fibres 

 terminated. It will be seen, however, that this argument does not touch the 

 character of the nerve-impulses in any two sets of nerves, and we have no 

 observations by which to decide whether the nerve-impulses passing along 

 the optic nerve-fibres are, for example, similar or dissimilar to those which 

 pass along the auditory fibres. 



If the nerve-impulses are always all alike, there seems no escape from the 

 inference that separate nerve-fibres convey the different sorts of impulses to the 

 cord. At the same time, it is just possible that the nature of the impulses and 

 of the resultant sensation is, in the nerves of cutaneous sensibility, determined 

 by the form of the peripheral stimulus, and that, as a consequence, different 

 branches of the same nerve-fibres may be conceived of as susceptible to differ- 

 ent forms of stimulation, and thus the two different sensations follow from the 

 partial stimulation of the same nerve-fibres. 



Pathway of Impulses in the Spinal Cord. The question arises how 

 these impulses are distributed among the afferent tracts which are recognized 

 in the cord, and whether these tracts form special paths for the impulses that 

 rouse the several sensations of pressure, temperature (heat and cold), and pain. 

 Since it is necessary to know the sensations of the subject, this problem can be, 

 in some ways, best studied in man. Here, owing to wounds or disease, it may 

 so happen that some of these sensations are lost or greatly diminished, and it 

 is to be determined whether this loss is constantly associated with the inter- 



