Chap. 28.] Structure of Nerves. <z S i 



Nervous cords have very little. elafticity compared 

 with fome other parts of the body. 



The Abbe Fontana has taken great pains to afcer- 

 tain the primitive ftructure of nerves. On examining 

 a number of nerves with microfcopes of low powers, 

 fo as not to magnify more than four or five diameters, 

 they always appeared to be furrounded with' white 

 fpiral bands not unlike the effect which would be pro- 

 duced by a ribbon twifted round a cylinder. The fpiral 

 bands were fometimes perfectly regular, fo as to be of 

 equal width along the whole length of the nerve exa- 

 mined, and to leave a fpace of a lefs bright colour of 

 the fame width between them ; at other times they 

 were irregular and crofled each other at uncertain dif- 

 tances ; this latter appearance, however, was found on 

 further examination to proceed from the nerve fub- 

 mitted to examination being compofed of many others : 

 for where he carefully feparated a nerve from thofe 

 which adhered to it, and examined it by itfelf, he al- 

 ways found the fpiral bands regular. He faw 1 thele 

 appearances very plainly in nerves not larger than a 

 haii;, with lenfes of very fmall pow-er, and was there- 

 fore perfuaded that this appearance of bands was not 

 an optical illufion. 



The lame nerve, however, which to the naked eye, 

 and by a lens of fmall power exhibited this appearance^ 

 when examined by a microfcope of high powers, ap- 

 peared to confift merely of parallel but twirling fibres. 



He next removed the cellular tifiue or fheath of a 

 nerve, without injuring ics texture ; but flill with a mi- 

 crofcope of high powers he could perceive nothing but 

 waving and twifting fibres, and nothing but fpiral bands 

 with the naked eye. After applying, however, to thefe 

 obfervations for two or three days, he found, that by 

 merely moving the reflecting mirror, he fometimes faw 



twifting 



