OF NATURAL HISTORY. 61 



moving machine. But fenfation, or the perception of plea- 

 iure and p:iin, is efrefled by organs of a peculiar kind. Thefe 

 organs are all comprehended under the general appellation of 

 the brain and nerves. 



Befide the bones of the cranium, the brain is invefted with 

 two membraneSj called dura and pia mater, becaufe they were 

 fuppofed by the Arabians to be the lource of all the other 

 meiTibranes of the body. Under the denomination of l>rain 

 are comprehended three diflincl parts, the cerci>ny7rj,tl\e wre- 

 helluiUi and medulla oblongata. The cerebrum is a foft medul- 

 lary mafsj fituated in the anterior part of the ikull, and divid- 

 ed, by a portion of the dura mater, into two hemifpheres. It 

 conliils of two fubftances, the cortical, which is greyifh, and 

 the medullary, which is ibfi;er, and of a very white colour. 

 The cerebellum is divided into two lobes, and its fubftance is 

 firmer and more compa-ri: than that of the cerebrum. It is 

 likewife compofeu of the cortical and medullary fubftances. 

 The reunion of the medallary fyoRances of the cerebrum 

 and cerebellum, at the bafts of the fkuU, forms the medid'a 

 cblongatay of which the fpinal marrow is a continuation. The 

 brain of the human fpecies is proportionally much larger 

 than that of quadrupeds. 



The brain and fpinal marrow are fuppofed to be the oricrin 

 of all the nerves or inftruments of fenfation. The nerves 

 are, in general, cineritious, fliining, inelaftic cords. But 

 they differ from each other in ftze, colour, and confiftence. 

 From numberlefs experiments and obfervations, it is unquef- 

 tionable, that the nerves are the inftruments both of ienfa- 

 tion and of animal morion. But, how thefe ,efte6ls are pro- 

 duced by the nervous influence is a difcovery ftill to be made. 

 The inquiry, however, has given rife to feveral ingenious 

 conjectures and hypothefes. Some phynologifts have main- 

 tained, that the nerves are folid cords, which may be divided 

 into an infinite number of minute filaments j and that, by 



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