PHYSIOLOGY. 37 



rately produce particular species, can unite in producing a sin- 

 gle sensation, which is always a neutral, or one different from 

 either of the separate sensations. 



LII. This union of impressions may take place, either when 

 the impressions are exactly synchronous, or when the one suc- 

 ceeds the other before the sensation of the first (XL VII I.) has 

 ceased. 



LIII. Though the motion excited in the sentient extremi- 

 ties, by impression, remains some time, as in XLVIIL, it 

 must be supposed to become continually weaker, till at length 

 it ceases altogether, and with it the sensation. 



LIV. The same impression soon repeated does not produce 

 the same strength of sensation as before. Hence, all new im- 

 pressions are, cceteris paribus, strongest ; and moderate impres- 

 sions frequently repeated produce no sensation, unless their 

 force is considerably increased. 



LV. Actions, which at first produced a sensation of consci- 

 ousness, as accompanied with volition, come, by repetition, to 

 be performed without any sensation ; or they produce it only 

 when they are performed with uneasiness, pain, or unusual 

 force. 



LVI. Impressions being given, their effects in producing sen- 

 sation are different in different persons, and in the same person 

 at different times. This must arise from some difference in the 

 state of the bodies acted upon, which may perhaps be referred 

 to the following heads : 1. The state of the common tegu- 

 ments, or other parts interposed between the impressing body 

 and the medullary substance of the sentient extremity. " No 

 where do external bodies come in contact with medullary sub- 

 stance without passing through other substances. Thus, in the 

 case of touch, the cuticle constantly intervenes ; there is some- 

 thing analogous in the tongue, there is a membrane in the nose, 

 the membrana tympani in the ear, and the various coats and 

 humours of the eye, through which the light passes before 

 actually striking the medullary substance. Now it is obvious 

 that these intervening substances may be, in different condi- 

 tions, fitted to transmit, more or less easily, the external impulse, 

 or entirely to interrupt it. In many cases the loss of sense is en- 

 tirely to be imputed to the imperfections of these media or in- 



