578 l>HILOSOPHICAL TRANSACTIONS. [aNNO 1733. 



is capable of exciting a contraction, supplying the want of that stimulus on the 

 inside, and also of causing a preternatural distension of the whole canal. The 

 contrary of all which the Doctor endeavours to prove. 



In order to this it is premised, 1. That the whole action of the nerves, whe- 

 ther in sensation or in muscular motion, is exerted at their extremities only. 

 2. That the sides of the nerves every where along their whole tracts, are en- 

 tirely insensible, and serve neither for sensation nor motion. 



The apparatus of nature towards both these actions makes this plain. To- 

 wards sensation we see, that the medullary substance of the nerves at their 

 extremities is divested of its coverings, which are processes of the dura and pia 

 mater, and ends bare in the form of small soft papillae, from their figure called 

 by anatomists pyramidales, on the surface of the cutis, covered over with the 

 cuticula, where they act their part in sensation, or feeling, tasting, and smell- 

 ing. The soft denudated branches of the optic nerve which compose the 

 retina, and what for the same reason is called the portio mollis of the 

 auditory nerve, the immediate instruments of seeing and hearing prove the 

 same. 



Again, it is the extremities of the nerves that enter with their coverings into 

 the muscle, and into each fibre of the muscle to which they belong; where 

 they deposit their contents, or act their part in muscular motion. 



But the sides of the nerves along their whole tracts are insensible, or void of 

 feeling; because their medullary substance, and its contents, which are the 

 only immediate instruments of sensation in them, are here covered with the 

 pia and dura mater, the last of vvliich is the strongest, densest, and most impe- 

 netrable membrane of the whole body, capable of defending and conveying the 

 tender medullary substance of the nerves and its contents, safe, unhurt, and 

 nndissipated to the several organs of sensation and motion, at their extremities, 

 the seats of their action. 



A further confirmation of this from experience, is the insensibility of the 

 side of a large visible branch of a nerve, which sometimes happens to lie bare 

 and exposed in a wound or ulcer, where it will bear the touch of the probe 

 without feeling, and occasions no more pain than in wounds and ulcers of 

 the same kind, where the nerves are not exposed, unless tlic investing mem- 

 branes, the dura and pia mater, be by any accident wounded, lacerated, or 

 corroded; in which case, the medullary substance being laid bare, exquisite pain 

 is felt, and verv severe symptoms ensue, which are hardly to be overcome, or 

 never so easily as by cutting the nerve quite through, so as that the extremity 

 may retire within the flesh, and the medullary substance be protected by it. 

 By which it appears, that the sides of the nerves are insensible or void of feel- 



