PHYSIOLOGY. 139 



body when the compression appeared very partial ; in several 

 instances, where the effects did appear, the cause did not ap- 

 pear, and there are various instances of very inconsiderable tu- 

 mours which produced palsy, apoplexy, and death. 



" 2. As the brain is more or less excited (CXXX.). 



u 3. According to the different causes of attention (XL VI I.). 



" Sensibility may, however, be considered also with regard 

 to its being erroneous or depraved. This consideration, I 

 have said, (see p. 50.) may be reduced to three heads: 1. 

 The perception of things which do not exist. When con- 

 sidering the states of the sentient extremities I have remark- 

 ed that this sometimes depends upon them ; but we may say 

 that it more generally arises from a state of the brain. When 

 the sensation is of a very simple kind, such as the starry 

 light which arises from rubbing the eyes, the muscae volitantes, 

 or a simple tinnitus aurium, it may depend upon the state of 

 the sentient extremities of the organ : but whenever the sen- 

 sationof the eye for example is of a more complex kind, 

 when we not only discern colours, but complex figures, &c. 

 few of these false representations depend upon the organ ; 

 thus also, when the false sensation appears in two or three or- 

 gans, when we not only fancy that persons speak to us, but see 

 them at the same time, the presumption is that such represen- 

 tations depend upon the brain. The inquiry how this state of 

 the brain operates, is in a great measure out of our reach ; but 

 there are some circumstances which we may consider with re- 

 gard to it. I have said (LXIV. LXIX.) that memory and 

 imagination differ in this, that though both renew combinations 

 of our ideas, in the one case we are conscious of the absence of 

 the object, while in the other we imagine that it is present. 

 Now the common account is that this difference depends upon 

 the vividness of the impression entirely : this supposition has 

 some foundation, as in delirium there is commonly an increased 

 impetus, tension, and excitement in the vessels of the brain ; 

 but in many instances of dreaming there is not the same founda- 

 tion, as they are of the most placid and calm kinds. I have 

 sought for another solution, therefore, and I think that imagin- 

 ation, or the perception of things which do not exist, is owing 

 to this, that the brain does not admit of any communication 



