PHYSIOLOGY. Ill 



on which they can operate only by the intervention of the brain. 

 Some of these causes operate with, others without sensation or 

 volition. 



CXX. In all or any of these cases in which the action of 

 the brain takes place, we cannot perceive the manner, that is, 

 the mechanical means, by which the several causes produce 

 their effects ; and we perceive only an institution of our Maker, 

 establishing a connexion between the several causes and the 

 motions that ensue. At the same time we, for the most part, per- 

 ceive, that the connexions established are suited to the purposes 

 of the animal economy, and particularly to the purpose of sup- 

 porting the system in a certain condition for a certain time, and 

 of averting what might hurt or destroy it. This constitution 

 of the animal economy, we call NATURE ; and every where in 

 the economy we perceive the vires conservatrices and me- 

 dicatrices naturce, so justly celebrated in the schools of physic. 

 " There have been theories to explain the operation of the 

 brain in our intellectual functions, but with no sort of success 

 in their application. There have been other attempts to explain 

 what might be viewed as mechanical communications by means 

 of the brain, but this with as little success as the other. Thus, 

 the tickling a man's side occasions laughter, and this may be 

 said to be owing to the respiration being particularly affected ; 

 but tickling elsewhere has the same effect, and none can say 

 why laughter is produced rather than coughing ; nor can we 

 assign the communication with the external skin, necessary to 

 give such a modification of respiration as occasions laughter 

 rather than coughing. When we consider sympathy, the com- 

 munication by nerves is by no means established. I formerly 

 said, that there is a peculiar connexion between the impres- 

 sions made upon our sentient extremities, and the particu- 

 lar sensations that our sentient organs produce. So with regard 

 to the whole of our volitions, there appears to be same. Why 

 does a sensation of the fauces excite thirst, and not a sensation 

 of dryness in any other parts ? Why does a sense of ridicule pro- 

 duce laughter, or an object of grief weeping ? For aught we 

 know laughter might have expressed grief and the contrary. But 

 we must learn the facts, 'the modus operand^ is entirely without 

 our reach. < At the same time,' I say, * we, for the most part, 



