156 PHYSIOLOGY. 



nervous fibrils, so wrapped up, proceed. But such an accident 

 seldom takes place ; and there are many instances of nerves 

 proceeding from the same origin without any such communica- 

 tion.'" And more commonly they may be explained, by sup- 

 posing the action of the impression to be general with respect to 

 the brain ; and that the affection of particular parts depends upon 

 the causes of determination (CXLII. CXLIII.). " I must 

 illustrate this by an example. The sight of grateful food occa- 

 sions the flow of saliva in a hungry person, and the appetite can 

 be renewed by recollecting what was formerly grateful ; and we 

 know that when the appetite is very sharp, a flow of saliva from 

 the mouth takes place. Now this is no instance of consent be- 

 tween the retina and the salivary glands ; the affection of these 

 glands is merely an expression of the appetite, and depends on 

 the general causes of communication ; for merely talking of very 

 grateful food will make the mouth water." 



When the actions of several parts, together or successively, 

 are necessary to produce one effect, these concur, though the 

 stimulus exciting the action of the brain is applied to one single 

 part only ; and commonly no other cause of communication can 

 be assigned but the several motions being necessary to the exe- 

 cution of the volition, propensity, &c. arising from the stimu- 

 lus. " Thus, by the laws of the economy, which we cannot 

 further explain, certain impressions produce certain volitions 

 and propensities, which, again, produce the action of certain 

 muscles. I cannot say why my will to lift a weight with my 

 hand determines the motion of the arm in a certain manner ; 

 but the motion is such that I cannot direct or even counteract 

 it ; according to the degree of effort, it will produce contractions 

 of my fingers, my wrists, and my shoulders, nay it will bring the 

 whole body into concurrence. If my finger is pricked, it will 

 have the same effect ; but it is not that there is a necessary 

 connexion between the nerve which was pricked, and the mus- 

 cles in my back and trunk, but that the painful sensation gives 

 me the volition of pushing away, which is necessarily attended 

 by all these different motions of muscles. Now, in explaining 

 vomiting, it is said that the stimulus applied to the stomach 

 touches nerves which have a sympathy with the diaphragm ; 





