PHYSIOLOGY. 31 



of the degree of motion excited ; but I am not conscious of the 

 several muscles which are put into motion to produce this ac- 

 tion. I am sensible of these only when they are exerted with 

 some unusual violence, or with spasmodic contraction. Inter- 

 nal motions also may come to be perceived when they are per- 

 formed with a spasmodic force. Thus, we take no notice of the 

 action of our heart, unless when any cause excites it in an unusu- 

 al manner, which we call a palpitation ; or if there is any in- 

 terruption or intermission in the pulse, which many persons can 

 mark in the heart. And so with respect to respiration, the usual 

 train of which we are not conscious of willing, or directing, but, 

 when it is performed with unusual frequency of force, we are." 

 6. The sensations arising from the diminution or absence of im- 

 pressions. " I will not say that darkness is visible, but it is a 

 positive sensation. The sensation of darkness or blackness is the 

 absence of impression ; it takes place when no rays of light fall 

 on our eyes, so that the sensation of consciousness may be here 

 separated from that of impression." 



Under each of these heads a great number of particular sen- 

 sations are comprehended, but not necessary to be farther spe- 

 cified here. 



Laws, or general Circumstances of Sensation. 



XLI. Of the four first genera (XXXVII.), the sensations 

 arising give no indication of the nature of the bodies acting on 

 our organs, or of the mode of their action ; and, when we other- 

 wise learn these circumstances, we can perceive no necessary 

 connexion between them and the sensations which they pro- 

 duce. But, from certain sensations of touch and consciousness, 

 we acquire the notions of solid figure, of motion, impulse, impe- 

 netrability, and the communication of motion, and consider the 

 sensations as exactly correspondent to the circumstances of ex- 

 ternal bodies. At the same time, as we know of no other ac- 

 tion of bodies on each other but that of impulse ; and as, in 

 the case of the sensations of the first four genera, we learn that 

 an impulse takes place, we have comprehended the whole under 

 the title of sensations of impression, and consider all of them as 

 perceptions of impulse " Whoever has the smallest tincture 



