282 Fontana's Observations on Nerves. [Bock IX. 



twifting fibres and fometimes- fpiral bands with the 

 fame lens. He is therefore under a neceffity of giving 

 up his fuppofed difcovery, and of allowing that the 

 fpiral bands were merely an optical deception. 



From his obfervations he can deduce no more than 

 that nerves are formed of a great number of tranfpa- 

 rent, homogeneous, uniform, and very fimple cylin- 

 ders. That thefe cylinders .are formed by an extremely 

 thin tunic, uniformly filled by a gelatinous trariiparent 

 humour, which is infoluble in water ; each of thefe 

 cylinders is covered by an external fheath, which is 

 compofed of a great number of twifting threads. 

 Many tranfparent cylinders conftitute a nerve which 

 is fcarcely vifible to the naked eye, and many of thefe 

 form the nervous cords which are feen in animals. 



The Abbe Fontana alfo fubmitted to the micro- 

 jfcope the medullary and cortical parts of the brains of 

 feveral animals. In thefe, however, the appearances 

 were pretty fimilar, and the fubftance of both appeared 

 to be organic, vafcular, tranlparent, and twifting, like 

 inteftirres. 



The nerves proceed from the encephalon and fpinal 

 marrow. Ten pair are ufually enumerated as arifing 

 from the former, and thirty from the latter. I fhall firft 

 defcribe the origin and courfe of thofe of the encephalon. 

 Anatomifts mention thefe in the order in which they 

 prefent themfelves when the brain is lifted from the 

 cranium; thofe which come- from the anterior part of 

 the cerebrum are therefore placed before thofe which 

 arife lower down from the cerebellum and medulla 

 oblongata. 



The firft pair of nerves is the olfactory, which pro- 

 ceed from the corpora ftriata ; they approach the cri- 

 briform plate of the os ethmoides, where they fplit into 

 a great number of filaments, which pafs through the 



per- 



