THE MEANING OF PERCEPTION 



165 



when the dorsal (or posterior) one is severed, there is loss of 

 sensation in some part of the body to which the mixed nerve is 

 distributed. When the ventral (or anterior) root is cut there is, 

 on the other hand, some degree of motor paralysis. Thus there 

 are two kinds of nerve fibres, some carrying impulses inwards 

 and others carrying impulses outwards. 



Later on still (in 1852) Waller's researches showed us the 

 anatomical basis for this conception of afferent and efferent 

 impulses. When a nerve fibre is severed from the cell out of 

 which it grows as an axon it dies, and its substance breaks down 

 and degenerates. The cells from which proceed the fibres 

 passing inwards through the dorsal root are in the sense orgaifs, 

 and so inwards from the place of section the nerve fibres de- 

 generate. On the other hand, the fibres passing out in the 



cord dorsal fse^r 



i-rom 

 Skin re 



7b ini^oluntqi'Y 



1 ^ — muscles 



voluntary 

 muscles 



FiG.^46. — Diagram of the Roots of a Spinal Nerve. 



A sympathetic ganglion is shown with the two rayni communicantes, one 

 branch going from the cord into the ganglion, and the other going out 

 from the ganglion to the involuntary muscles (those of the alimen- 

 tary canal and bloodvessels, in the main). 



ventral root proceed from cells in the grey matter of the spinal 

 cord, so that when they are cut through they degenerate on the 

 side of the place of section away from the spinal cord. The 

 result of this discovery was that it became possible to trace the 

 paths in the mixed nerves, in the spinal cord, and in the brain 

 along which impulses travelled, and so the results suggested in 

 Chapters VI. to VIII. were obtained (mainly by experiments on 

 animals). It was also found that many of the fibres in a nerve 

 distributed to a muscle came from the latter — that is, they carried 

 afferent impulses from receptors there. Later, again (almost in 

 our own time), Sherrington's beautiful researches showed that 

 . all movements involved antagonistic muscles — that is, when one 

 (flexor) muscle contracted, another (extensor) one relaxed, and 

 this in consequence of one efferent impulse proceeding outwards 



