APPENDIX TO MEMOIR OF PELTIER. 177 



the arrangement in a straight serial line. The sheath is shortened in obeying 

 this new arrangement ; it fohls slightly on itself, like the finger of a glove whose 

 two ends are pressed nearer together. The elasticity of the sheath, however, 

 renders this connigation very difficnlt to he perceived. 



On the structure oftJie nerves. — When Charles Bell had published his treatise 

 on the distinction of the nerves of the face into nerves of movement and nerves 

 of sensation, Magendie proceeded to inquire whether there were not something 

 analogous in the rachidian nerves, and soon thereafter proved, in effect, that the 

 posterior roots of these nerves presided over the sensibility, while the anterior 

 roots governed the power of movement. It was natural to suppose that these 

 two sorts of nerves had a different structure and constitution. Peltier applied 

 himself to this interesting qnestion, and we will recall the principal facts which 

 he made public* 



The nei'ves of sensibility have not a texture similar to those of movement, 

 and moreover each of them in particular varies according to the proximity of its 

 insertion in the organ or of its exit from the cerebro-spinal centre. 



In removing further from the cerebro-spinal centre, the cellular tissue of the 

 nerves increases and becomes more resistant ; it circumscribes more and more the 

 medullary pulp, and in the end forms for it distinct sheaths. At first there are 

 but small portions of this pulp thus circumscribed and enclosed in the sheatlis; 

 the rest surrounds them and fills the interstices which separate them. The 

 number of these sheaths continually increases, and the free pnlp diminishes in 

 the same proportion. The nearer we approach the termination of the nerves, the 

 more glutinous does this pulp become and the greater the cohesiveness it acquires.. 



The nerves which are ramified in the muscles are formed of tubes of about 

 Y^Q of a millimetre ; the membrane which constitutes them is of little consist- 

 ency; at the least pressure it yields unequally, and the medullary substance 

 which it contains forms varicosities. The nearer the periphery, the fewer the 

 varicosities, because the sheath becomes more resistant and the pulp diminishes. 

 These tubes or nervous fibrils, however, always preserve a considerable part of 

 their globules in line, whatever the pressure exerted on them. Towards their 

 insertion they are finer, more regular, and more numerous ; the globules of the 

 pulp are there better aligned, their position is fixed, pressure no longer displaces 

 them, and these nervous fibrils might be readily confounded with the muscular 

 fibrils, if the transverse lines found in the latter were not wanting. 



Arrived at the muscle to which it is destined, the nervous filament sends forth, 

 at variable distances, bundles of elementary fibrils which have become extremely 

 thin. They are in diameter about -g-i^ of a millimetre, and are only formed of 

 a series of contiguous globules ; scarcely does pressure any longer discover a 

 little free pulp in their interstices. These bundles of nervous fibrils are dis- 

 persed over all the adjacent muscular fibrils, in the midst of which they success- 

 ively disappear, without our being able to see how they terminate. It might 

 almost be believed that the muscular fibril is, as regards a part of, its substance, 

 but a contiimation of the nervous fibril. 



The nerves of sensibility have a different constitution from the preceding. 

 They contain less of the nervous pulp in a state of semi-fluidity ; on compression 

 no varicosities are produced ; their fibrils are more tenuous ; they have, at first, 

 a diameter of from gJ^ to -^^-^ of a millimetre, but towards the organ in which 

 they are inserted of not more than from loVo ^^ itVo ^^ ^ millimetre. Their 

 globules are much smaller, being not larger than about y^'o o ^^ ^ millimetre ; 

 they are regularly aligned, and pressure does not displace them. These 

 fibrils often cross one another in their progress. A certain number of them, 

 united in little bandlets, form, in crossing, lozenges elongated at the point of 

 their intersection ; these bands are strongly adherent, and cannot be detached 

 but by tearing them. 



* Journal VInstitut, 1839, t. vii, p. 113. 



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