166 HISTORY OF PHYSIOLOGY. 



lucid, and excellent notion of the Cambridge teacher was not accepted 

 with greater alacrity, and farther unfolded by his contemporaries." It 

 has, however, since been universally adopted. 



But though the discrimination of muscular irritability as a peculiar 

 power might be a useful step in physiological research, the explanations 

 hitherto offered, of the way in which the nerves operate on this irrita- 

 bility, and discharge their other offices, present only a series of hypo- 

 theses. Glisson 15 assumed the existence of certain vital spirits, which, 

 according to him, are a mild, sweet fluid, resembling the spirituous part 

 of white of egg, and residing in the nerves. This hypothesis, of a very 

 subtle humor or spirit existing in the nerves, was indeed very early 

 taken up. 16 This nervous spirit had been compared to air by Erasis- 

 tratus, Asclepiades, Galen, and others. The chemical tendencies of the 

 seventeenth century led to its being described as acid, sulphureous or 

 nitrous. At the end of that century, the hypothesis of an ether attracted 

 much notice as a means of accounting for many phenomena ; and this 

 ether was identified with the nervous fluid. Newton himself inclines 

 to this view, in the remarkable Queries which are annexed to his 

 Opticks. After ascribing many physical effects to his ether, he adds 

 (Query 23), "Is not vision performed chiefly by the vibrations of this 

 medium, excited in the bottom of the eye by the rays of light, and 

 propagated through the solid, pellucid, and uniform capillamenta of 

 the nerves into the place of sensation ?" And (Query 24), " Is not 

 animal motion performed by the vibrations of this medium, excited in 

 the brain by the power of the will, and propagated from thence through 

 the capillamenta of the nerves into the muscles for contracting and 

 dilating them ?" And an opinion approaching this has been adopted 

 by some of the greatest of modern physiologists ; as Haller, who says," 

 that, though it is more easy to find what this nervous spirit is not than 

 what it is, he conceives that, while it must be far too fine to be per- 

 ceived by the sense, it must yet be more gross than fire, magnetism, 

 or electricity ; so that it may be contained in vessels, and confined by 

 boundaries. And Cuvier speaks to the same effect :" " There is a great 

 probability that it is by an imponderable fluid that the nerve acts on 

 the fibre, and that this nervous fluid is drawn from the blood, and 

 secreted by the medullary matter." 



Without presuming to dissent from such authorities on a point of 



" Spr. iv. 38. w Haller, Physiol. iv. 365. 



ir Physiol. iv. 381, lib. x. sect. viii. 15. 18 Rtgne Animal, Introd. p. 30 



