TRANSFORMATION AND EQUIVALENCE OF FORCES. 221 



Besides the correlation and equivalence between exter- 

 nal physical forces, and the mental forces generated by them 

 in us under the form of sensations, there is a correlation and 

 equivalence between sensations and those physical forces 

 which, in the shape of bodily actions, result from them. 

 The feelings we distinguish as light, heat, sound, odour, 

 taste, pressure, &c., do not die away without immediate re- 

 sults; but are invariably followed by other manifestations 

 of force. In addition to the excitements of secreting organs, 

 that are in some cases traceable, there arises a contraction of 

 the involuntary muscles, or of the voluntary muscles, or of 

 both. Sensations increase the action of the heart slightly 

 when they are slight ; markedly when they are marked ; and 

 recent physiological inquiries imply not only that contrac- 

 tion of the heart is excited by every sensation, but also that 

 the muscular fibres throughout the whole vascular system, 

 are at the same time more or less contracted. The respira- 

 tory muscles, too, are stimulated into greater activity by 

 sensations. The rate of breathing is visibly and audibly 

 augmented both by pleasurable and painful impressions on 

 the nerves, when these reach any intensity. It has even of 

 late been shown that inspiration becomes more frequent on 

 transition from darkness into sunshine, a result probably 

 due to the increased amount of direct and indirect nervous 

 stimulation involved. When the quantity of sensation is 

 great, it generates contractions of the voluntary muscles, as 

 well as of the involuntary ones. Unusual excitement of the 

 nerves of touch, as by tickling, is followed by almost incon- 

 trollable movements of the limbs. Violent pains cause vio- 

 lent struggles. The start succeeds a loud sound, the wry 

 face produced by the taste of anything extremely disagree- 

 able, the jerk with which the hand or foot is snatched out 

 of water that is very hot, are instances of the transforma- 

 tion of feeling into motion ; and in these cases, as in all oth- 

 ers, it is manifest that the quantity of bodily action is pro- 

 portionate to the quantity of sensation. Even where from 



