SECT. I.] ACTION OF THE NERVES ON IMPRESSIONS. 407 



aptitude of the nerves to receive impressions, and when received 

 of transmitting them either way with great rapidity, appears to 

 be that called the vis nervosa of the nerves, which is also cor- 

 rectly designated, the sensibility or mobility of the nerves, and 

 which is also well designated by Unzer, " corporeal sensation 

 without concomitant perception.^^ 



This property of the nerves does not depend solely on their 

 medullary pulp, (which possibly is capable of some slight vibra- 

 tion, or oscillation, although the nerves do not appear at all 

 tense,) but it appears, as I have already observed in § 3 of 

 the preceding Chapter, to be rather some other principle added 

 to the medullary pulp, the conjunction of the two constituting 

 the whole vis nervosa j and possibly the diligence of the very 

 sagacious observers of nature may discover whether that prin- 

 ciple be electricity, or phlogiston, or some species of air, or the 

 matter of light, or a something compounded of these. That other 

 principle, whatever it may be, seems to come to the nerves with 

 the arterial blood, by means of the numerous blood-vessels which 

 accompany the nerves of the whole body throughout their whole 

 course ; or to be attracted from the air through inorganic pores; 

 or in both these ways, and not to be sent into the nerves 

 from the brain, as its only source, although the brain itself 

 appears to acquire a suitable portion of the same principle 

 through its own vessels. For, as I have before shown, the 

 nerves when separated from the brain have equally vis nervosa 

 as the nerves in connection with the brain, and in proof 

 hereof may be mentioned the nerves of acephalous foetuses 

 and of brainless animals, which are endowed with the vis 

 nervosa, although they could not possibly derive it from a brain. 

 Nevertheless, a certain cohesion of the medullary pulp of the 

 nerves is necessary to the vis nervosa, because if we so injure 

 the pulp of a nerve by strong compression, that the connection 

 of its globules is destroyed, and their relations broken up, the 

 vis nervosa ceases in the portion of the nerve thus compressed, 

 neither can impressions be propagated through it, nor can that 

 portion of the nerve produce motion or sensation, if pricked or 

 irritated. 



Although a nerve be necessary to sensation and motion, it does 

 not excite motion or feel alone, but feels by means of the brain, 

 which, when an impression made on a nerve is brought to it, 



