108 Alison on the Theory ascribing Secretion 



2. That many muscles are directly excited to contraction by 

 causes acting on the nervous system. And, 



3. That the contractile power of all muscles may be much 

 and variously modified, and in some instances destroyed, by 

 causes acting on the nervous system*. 



If these principles be correct, we see the necessity, in all in- 

 vestigations concerning the nervous system, of keeping steadily 

 in view a principle which no train of reasoning is necessary to 

 establish, but which has often escaped the attention of those 

 engaged in such inquiries. 



•That changes in the nervous system affect muscular action in^otk these 

 ways was distinctly pointed out by Haller, in the same paper in which he 

 controverted the doctrine of the dependence of muscular irritability on ner- 

 vous influence, although tlie French reporters on Le Gallois's work seem to 

 have supposed that, according to hira, changes in the nervous system can 

 only act in one way upon muscles, viz., by directly stimulating them, and 

 therefore that, in his apprehension, the nerves of the heart must have been 

 absolutely useless. " Un jour," says he, " pent titre. Ton reduira 1' usage 

 " des nerfs, par rapport aux muscles, a leur porter, de quelque fa<;on que 

 " la chose se fasse, 1' impression des volontOs del'ame, et -a. augmenter cette 

 " tendance naturelk, que k$ fibres ont d^jii par elles inemes, a sc contradir **." 



And again, " minime improbabile est, etiam cordi per nerves vim motricein 

 " accedere, qua; uiotum, a natura irritobili pendentem, fortiorem et celeriorem 

 " efficiattt." It is impossible to express more clearly that the muscularpower 

 of the heart, altliough dependant only on its irritable nature, or as Dr. _W. 

 Philip expresses it, perhaps less correctly, on its mechanism, is liable to in- 

 crease, and we may add, to diminution, from causes acting on the nervous 

 system. I apprehend, therefore, not only that Haller has not overlooked the 

 uBe of the nerves of the heart, but that he has atated their use more correctly 

 than any physiologist since his time. 



The experiments of Dr. Philip have illustrated more fully than those of any 

 other physiologist these two dj^erent modes, in which changes in the nervous 

 system affect muscular action. The general results of his observations, and 

 of those of Haller, Bichat, and others on this subject, may be stated to be, 

 thatthe^rsf mode of action, that is, the direct excitation of muscular fibres 

 to contraction by impressions made on the nervous system, is confined to the 

 vohintary muscles ; and that the second mode of action, that is, the alteration 

 of the vital power, or tendency to contraction of muscular fibres by im- 

 pressions made on the nervous system, is chiefly exerted on the inwluntarji 

 muscles. 



Dr. W. Philip has indeed recorded in the second chapter of the second 

 part of his \vork, some very important experiments which he considers as 

 ** Dissert. surl'lrritabUiti, p. 52. 

 1 1 Ekm. Physiol. Vol. I. p. 493. See also Vol. IV. p. 51C. 



