102 ZOOLOGY. 



in the hemispheres of the cerebrum, to which the sensations 

 arc transmitted by the nerves. 



200. Neither ought it to be forgotten that each nervous 

 fibril is quite distinct from one extremity to another; and to 

 this must be ascribed the distinctness in our sensations, and 

 t he referring them always to their peripheral extremities. After 

 amputation, the pains which occasionally continue are con- 

 stantly referred to the foot or hand which is no longer 

 present. 



201. Nerves of Sensibility or Sensation. All nerves 

 have not the power of transmitting sensations to the brain ; 

 some, on the contrary, are clearly nerves of motion, whether 

 acted on by our will or excited by other means. Some 

 nerves, as the optic, transmit only the impressions received 

 from colours ; to other stimulants this nerve is insensible. 



202. Modifications of t/ie Sensibility. To these modi- 

 fications of the sensibility we owe the five senses ; the senses 

 of touch, taste, smell, hearing, seeing, are so many distinct 

 faculties putting us in relation with the various qualities of 

 the external world. To the first pair of nerves belongs the 

 sense of odours ; by the second pair we perceive coloured 

 bodies ; the portio mollis of the seventh pair, by some called 

 the eighth pair, is the instrument by which the sensation of 

 sounds is conveyed to the brain ; and by means of a branch 

 of the fifth pair we perceive the sapid qualities of objects ; 

 whilst the spinal nerves distributed to the fingers and toes, 

 endow their extremities with fine tactile powers. 



203. The roots of the spinal nerves being double, a 

 double function had been long suspected ; this Avas put beyond 

 dispute by Bell and Ma^endie,* who showed that by the 

 posterior roots the sensations travel to the brain, whilst by 

 the anterior the power of motion travels to the muscles. 

 Some differences have been observed in various parts of the 

 spinal marrow ; the sensibility is acute in the dorsal aspect 

 of the organ, and much more feeble anteriorly. 



204. (iain/Honary System. This system is but little, if 

 at all, sensible to ordinary stimulants ; neither cutting nor 

 pinching affects it. In the healthy state, the organs to 

 which the nerves of this system proceed are in like manner 

 but little, if at all, sensible; but when diseased their sensi- 

 bilities become highly exalted. 



First by Mr. A. Walker.-R. K. 



