VARIETIES OF THE SENSES. 343 



amongst the nerve cells of the sensorial centre, but must refer them 

 to the preliminary formation of definite patterns of images upon the 

 retinal structure. The same objection may be made to instances of 

 the sensation of light being caused by pressure on the brain, of sparks, 

 of buzzing or other noises, and of creeping sensations in the skin, 

 being produced by narcotic agents, in all which cases, the recipient 

 organs are still connected with the sensorial nervous centres. The re- 

 sults of the application of a common external stimulus, such as a blow, 

 pressure, or electrical shocks, to the various sensory organs, all of 

 which undoubtedly produce the sensations of light and color in the 

 eye, loud or ringing noises in the ear, a salt or acid taste in the tongue, 

 and shock or pricking in the skin, are open to the same objection. As 

 to the effects of direct stimulation of the trunks of the gustatory, olfac- 

 tory, or auditory nerves, by electricity or mechanical means, nothing 

 is known; direct irritation of the optic nerve, is said to produce not 

 pain but a sensation of light; no such experiment, however, has yet 

 been made, after the complete removal of both eyeballs, and therefore, 

 even if one eye had been removed, some recurrent effects may have 

 been produced through the retinal elements of the remaining eye. 



The existence of a special susceptibility in the internuncial nerves 

 of the different senses, is inferred from similar, but equally defective, 

 evidence to that adduced in regard to the supposed distinct endowments 

 of the sensorial centres. It has even been assumed that they possess 

 not only a special irritability as regards certain heterologous stimuli, 

 but special qualities which can generate, under ordinary or homologous 

 stimulation, peculiar sensations in the several sensorial centres ; but 

 the proofs adduced are of the same imperfect character ; and some dis- 

 turbance or altered condition of the recipient apparatus still in con- 

 nection with the nerves, may have been the cause of the -specific reac- 

 tion of the nerve and its nervous centre. The well-known phenomenon 

 of sensations referred to the lost toes or fingers, after amputation of 

 the limbs, presents an example of localized sensation, dependent on 

 irritation of a nerve-trunk ; but such sensations are not special, or such 

 as apparently require a recipient organ to excite them. They resemble 

 rather the sensation of pins and needles in the little finger, produced 

 by a blow upon the ulnar nerve at the elbow, and consist of modifica- 

 tions of pain rather than of definite tactile impressions, or impressions 

 of temperature, such as can only be produced through the cutaneous 

 organs. 



The phenomena of special sensation and its varieties may perhaps 

 be accounted for without assuming the existence of absolutely distinct 

 physiological endowments in the internuncial nerves and nervous cen- 

 tres. The conducting and recipient powers of these may be the same 

 or fundamentally alike ; their structure, at least, presents no recog- 

 nizable difference sufficient to account for any absolute difference of 

 endowment ; any disturbance in their molecular constitution, produced 

 by a sensory impression, may be supposed to involve the same vito- 

 chemical and vito-physical changes in the substance and condition of 

 the nervous elements, whether characterized by oxidation, 'heat, or 

 internal motion ; these changes may present merely correlated differ- 



