74 PHYSIOLOGY. 



other part of the nervous system, though both kinds of fibres, by 

 XCIV., consist of the same kind of matter. "Nothing appears 

 so soft and tender, and so little coherent, as medullary fibres in 

 every part of the nervous system, and it may appear difficult to 

 suppose that they consist of the same kind of matter as muscles ; 

 but if these fibres are capable of having that fluid, which ope- 

 rates by increasing the force of cohesion, accumulated, it will 

 show why these fibres show more cohesion than the medullary 

 fibres do in any other part. I have been at pains to inculcate 

 this point, that the matter in the medullary substance and in 

 the muscular fibres, is of one and the same kind. 



" Dr. Haller has taken a partiality for a doctrine of which 

 he thinks himself the author, viz. that the vis nervea and the 

 vis insita of muscular fibres are totally distinct powers. I 

 have touched the matter before, but I have not taken notice of 

 the arguments of Dr. Haller in favour of the contrary opinion ; 

 and I am now in a condition to enter into the controversy with 

 Haller. In his Primce Linece, 403, he establishes the vis 

 nervea, and in 404 he proceeds in this manner : ' Haec vis 

 non est eadem cum vi insita ;' and his reasons are ' Forinsecus 

 ad musculum advenit, in quo altera inquilina habitat' the 

 nervous power comes from without, the inherent power is con- 

 stantly resident in the muscular fibre. Various hypotheses with 

 regard to muscular motion have assumed the nervous power 

 flowing in larger quantity into the muscular fibre ; at the same 

 time none of them have supposed that the muscular fibre is in- 

 dependent of the nervous power ; but they always assumed that 

 a portion of that power is constantly there, upon which its con- 

 tractility depends, and that the excitement of the muscular 

 fibre may be increased by an addition of more nervous power. 

 It is, agreeably to the doctrine we have laid down, very proba- 

 ble that the excitement we speak of is in common to the medul- 

 lary fibre every where, and that it may receive an addition from 

 the nerves, without however being a different matter. But his other 

 argument is, that the nervous power: < Cum vita destruitur, a qua 

 altera, per certa experimenta diu superest. 1 I have prepared for 

 this argument by what I have said in XC VI. From the experi- 

 ments of Dr. Smith, which I formerly quoted, it appears that the 

 nervous power remains as long as the inherent power; that so long 



