138 PHYSIOLOGY. 



CXLI. The effect of sensation is commonly that of exciting 

 the action of the brain ; and this is generally according to the 

 degree of volition produced under the different circumstances 

 mentioned in CXIX. 



CXLI I. The action of the brain excited by volition or other 

 causes, is determined to particular parts of the body most con- 

 stantly by the connexions established in the system (CXX.), 

 but also occasionally by acquired habits, or the greater mobility 

 of certain parts. " I know a person who lost a prodigious 

 quantity of blood by a wound of the arm, by which he was 

 considerably weakened. Whatever emotions of the mind af- 

 fected this person, the arm was seized with a considerable tre- 

 mor ; a weakness and mobility had been produced in this part 

 which determined these effects. Thus, persons who have ac- 

 quired a particular irritability and mobility of the stomach, are 

 made to retch and vomit by slight emotions ; and, in like manner, 

 persons who have acquired a particular weakness of the bowels, 

 are sent to their closet by every emotion of the mind. There 

 are instances of mobility in certain parts ; and whatever affects 

 the system more generally, has its effect especially on that 

 weak part." 



" The different morbid states of the brain are not easily ascer- 

 tained ; but they are to be attempted, and we must say what 

 we can regarding them ; and I propose to do so by considering 

 how it may be in different states affecting sensation and mo- 

 tion. 



" I. With regard to sensation; the state of the brain giving 

 sensibility in a greater or less degree is different : 1. As it is more 

 or less free (XXX.). If from any cause the freedom of its 

 motions is interrupted, no sensation can take place : we have 

 frequent instances of this where the brain is under compression, 

 the most frequent cause of this interruption, from external force 

 or fluids effused in the ventricles or on the surface. But it is 

 difficult to say what degree of compression is necessary to take 

 away sensation and motion. Experiments have been made 

 upon dogs and other animals, from which it appears that a 

 strong and considerable degree of compression is necessary; 

 but we have seen the loss of sense and motion in the human 



