364 ANNUAL OP SCIENTIFIC DISCOVERY. 



and here physiology takes up the theme, and endeavors to add new light to 

 the object. It may not be so sure as anatomy ; it is a progressing, a new 

 science ; but how great interest is not felt in its least discoveries, as it tries 

 to explain the operations of the organs, or at the least to exhibit the use of 

 their different parts ! 



One of the most important facts discovered by the experiments of vivi- 

 section is the unquestionable difference existing between two sorts of ner- 

 vous fibres, these being exclusively affected to sensation, and those to mo- 

 tion. The reader will remark, the word fibre, and not nerve, is used ; for, 

 by a very remarkable singularity of organization, most of the nerves which 

 are ramified in the different parts of the body are mixed nerves ; i. e., 

 groups of two sorts of fibres so confounded together they cannot be sepa- 

 rated ; it is only in a very limited portion of their route the fibres of the 

 same species are assembled together. Take at will in the body of a man 

 any nerve large enough to be followed easily towards. its origin ; the stu- 

 dent will be led to the spinal marrow, their common origin, and he will 

 recognize that the nerve is implanted in it by two roots (and not very large) 

 placed behind each other. Of these two roots, the anterior is formed of 

 fibres used to excite motion ; in the posterior root, on the contrary, all the 

 fibres are exclusively destined to sensation. Pinch the former in a living 

 animal, there will be convulsive movements ; irritate the second, and the 

 animal shows by his cries the pain he suffers ; but if the student tries this 

 experiment some centimeters only farther from the conjunction of the two 

 roots, both of these effects will be produced together, for he will be at the 

 same time operating on both species of fibres. 



This admirable discovery of Sir Charles Bell has engaged physiologists 

 to endeavor to ascertain whether the marrow itself is not formed of motive 

 and sensitive parts. Their conclusions clash. It appears, indeed, the mo- 

 tive power belongs to the anterior cords, and sensibility to the posterior 

 cords ; but these characteristics are not so distinctly defined as in the pair 

 of nerves which emanate from the same cords ; for the two opposite 

 regions of the marrow are far from being anatomically isolated, (as the two 

 species of roots are ;) besides, the whole organ is subject to reflex actions, 

 which, while they do not equal those of the brain, make the marrow some- 

 thing more than a nerve, and exhibit an intimate union between the parts ; 

 lastly, there exists in the centre of the marrow a grayish substance, 

 whose functions had not heretofore been defined, and which might lead to 

 difficulties in the experiments which could not readily be understood. This 

 is the leading object of M. SchifFs memoir. After becoming persuaded 

 that the posterior cord, raised and detached from the rest of the marrow 

 for a certain distance, felt and transmitted sensitive impressions, he made 

 on the grayish substance a series of experiments, which demonstrate that, 

 like the whitish susbtance, it transmits the impression of pain : but when he 

 irritated the substance itself, he ascertained that it remained completely 

 insensible. 



This was the most conclusive experiment he exhibited before the Academy 



