CHAP. XII.] SYMPATHETIC SENSATIONS AND MOTIONS. 391 



kind would produce, and therefore gives rise to effects of the same 

 nature as those resulting from that mental change. Thus the smell 

 of savoury food excites in the mind the idea of food, which in a 

 hungry man would, if it occurred spontaneously, occasion a flow of 

 saliva. And the odour which occasions syncope, creates in the mind 

 an emotion of disgust, which, if it arose independently of the physi- 

 cal impression, would affect the heart through the centre of emotion. 

 It is plain, however, that that portion of the nervous centre which 

 is affected in such cases, must have a direct influence upon the parts 

 in which the sympathetic phenomena appear ; and this through 

 commissural fibres, or the continuity of its gray matter with that 

 of the centre from which its nerves immediately spring ; thus, in 

 the instances referred to, the centre of sensation, which is first 

 affected, is, through the medulla oblongata, connected with the sali- 

 vary glands by the fifth nerve, and with the heart by the vagus. 



We derive an explanation of the third class of sympathetic phe- 

 nomena from the known laws of sensitive and motor nerves. It is 

 known that stimulation of a sensitive nerve at its origin, or in any 

 part of its course, will give rise to a sensation which will be referred 

 to the peripheral extremity of the stimulated fibres ; and that a 

 stimulus applied to a motor nerve causes a change in it which 

 spreads peripherad from the point stimulated, and therefore affects 

 the muscular parts with which it is connected. It is known, also, 

 that a sentient nerve may excite a motor or sensitive nerve which is 

 implanted near to it in the nervous centre doubtless through the 

 change which it produces in that centre ; nor can it be doubted 

 that a sensitive nerve may receive such a powerful stimulus as to 

 exalt the polar force of a large portion of the nervous centre in the 

 neighbourhood of its insertion, and thus to excite a similar change 

 in all the nerves, whether motor or sensitive, which are connected 

 with it. Thus, according to the intensity of the original stimulus, 

 there will be a radiation of nervous force from the centre, either in 

 one or two motor or sensitive nerves, or in several such ; and the 

 number and variety of the sympathetic phenomena will thus depend 

 on the intensity and extent of the change in the nervous centre 

 excited by the primary stimulus. 



To explain, then, the phenomena of sensation and motion under 

 consideration, we must determine the individual nerves affected in 

 each instance, and ascertain what connexions they have with each 

 other. We learn from anatomical investigation, that, although nerves 

 anastomose with each other in their distribution, this anastomosis 

 is by no means of that kind which would justify the surmosition that 



