PHYSIOLOGY. 127 



not stimulant, although others are. There is, however, no doubt 

 of the fact, that every impression which produces a sensation, 

 is a means of preventing sleep, and of causing waking ; and 

 you will easily recall what I said (CXIX.) with regard to this 

 being the source of activity in the whole system. 



" I have added ' impressions analogous to those which pro- 

 duce sensation? This will be difficult to most readers, but it may 

 be made easy. A great number of motions go on in the internal 

 parts of our bodies, such as the dilatation of the heart by the 

 venous blood, and the motion of food and air in the alimentary 

 canal, which excite its action ; and so, in the other organs of se- 

 cretion also, there are a great many impressions going on that 

 produce no sensations, of which we are no ways conscious, but 

 which are causes of impulse ; and many of these internal impres- 

 sions, of which we are not conscious, are causes of waking. 

 So persons who labour under a weakness of the stomach, as I 

 have done for a great number of years past, know that certain 

 foods, without their being conscious of it, prevent their sleeping. 

 So I have been awaked a hundred times at two o'clock in the 

 morning, when I did not feel any particular impression ; but 

 .1 knew that I had been awaked by an irregular operation in 

 that organ, and I have then recollected what I took at dinner, 

 which was the cause of it. Dr. Haller is liable to the same com- 

 plaint; and, in his larger work especially, he gives the particu- 

 lars of his own case, and to the same purpose that I have done, 

 as he learned it from his own experience. It is very neces- 

 sary to admit and observe these causes of waking. It was 

 necessary to add, < all sensations which lead to thought and 

 action.' It is a sort of exception when I said that certain single 

 sensations, or sensations which lead to no thought, are ever 

 causes of sleep. I need not say any thing towards explaining 

 this, viz. that it is sensations which especially lead to thought; for 

 if they lead to no observation of their relations, or how far 

 their relations may extend, they are, as with respect to certain 

 sounds, vox, et prceterea nihil; and it is those sensations which 

 lead to reflection, to intellectual operations, or those that are ac- 

 companied with desire and aversion, that especially lead to 

 waking and prevent sleep. Thus there is a certain activity in 

 dreaming which will produce the waking state. 



