0(5 PHYSIOLOGY. 



nerve, it has the effect of interrupting all power of will over the 

 muscle. We might indeed suppose that the brain has nothing 

 to do with this, and that it is owing to a superior portion of the 

 nerve ; but the contrary appears when the ligature removed to 

 a part nearer the brain has the same effect. But the experi- 

 ment may be diversified. We know that besides the power of the 

 will there are other powers which excite the contraction of the 

 muscles, and which should have been described in XXXII. 

 when I said that the example adduced (sensation and volition) 

 is not the only way of communication between the parts of the 

 nervous system. Thus, the pricking of a needle between the 

 ligature on the nerve and its extremity produces a contraction, 

 while the will has no such power ; but if we carry a puncture 

 between the ligature and brain we can produce no such action 

 of the muscle. If a ligature or compression be applied to a 

 nerve of sense of the latter of which we have frequent instances 

 in the optic nerves the impression, of light for instance, does 

 not produce its peculiar sensation ; but if we can go beyond the 

 ligature, and apply a puncture between it and the brain, this 

 will occasion a sensation, whilst there is no sensation by any 

 application between the ligature or compression and the organ. 



" This, I think, points out clearly that, from the part on which 

 the impression is made, some motion is excited, and that this 

 can be carried to the other extremity of the nerve. I say, 

 therefore, ' we presume that sensation arises only in conse- 

 quence of external impulse producing motion in the sentient 

 extremities of the nerves, and of that motion's being thence pro- 

 pagated along the nerves to the brain.' It is in consequence of 

 its arriving in the brain only that sensation is produced ; so that 

 sensation is a function of the brain alone : the soul perceives 

 in the brain only, and not in the sentient extremities."" 



XXXIV. From what is now said, we perceive more distinct- 

 ly the different functions of the several parts of the nervous 

 system, as distinguished in XXIX. 1. The sentient extremi- 

 ties (XXIX. 3.) seem to be particularly fitted to receive the 

 impressions of external bodies; and, according to the difference 

 of these impressions, and of the condition of the sentient ex- 

 tremity itself, to propagate along the nerves motions of a de- 

 termined kind, which, communicated to the brain, give occasion 



