ACTIONS OF NERVOUS SYSTEM. 79 



be described it has been fully proved that these opposite 

 changes are conducted by two different sets of fibres ; one 

 conveying to the central organs those which originate in the 

 circumference; and the other conveying to the circum- 

 ference those which originate in the centre ( 451). The 

 transmission of these changes is completely interrupted by 

 division of the nervous trunk, or by pressure upon it ; and it 

 sometimes happens that one set of conducting fibres is thus 

 affected, whilst the functions of the other are not impaired ; 

 so that a limb may retain its sensibility and yet be totally 

 destitute of the power of motion, or may be completely 

 obedient to the will though totally destitute of sensibility. In 

 Yertebrated animals, we find some nerves in which there is 

 only one set of fibres, so that the trunk is only sensory or only 

 motor ( 459); but in general, the two sets are bound up 

 together in the same sheath. 



63. The motor fibres may be considered as originating in 

 the vesicular substance of the central organs, and as termi- 

 nating in the muscles- ; the power which is generated in the 

 former being conveyed by their means to the apparatus through 

 which it operates to produce mechanical motion. When the 

 nerve-trunks reach the muscles, they divide into branches 

 which penetrate their substance, and these again subdivide 

 and ramify minutely, so that at last the fibres may often be 

 observed running singly, passing amongst the muscular fibres, 

 but not appearing to penetrate their tubular sheaths. These 

 terminal fibres seem often to double back upon themselves, so 

 as to form loops, either re-entering the branch from which 

 they issued, or connecting themselves with some neighbour- 

 ing branch ; so that the ultimate distribution of the motor 

 nerves in the muscular substance, is a sort of plexu,s or net- 

 work. The sensory fibres, on the other hand, may be con- 

 sidered as originating in the sensory surfaces, such as the 

 skin, the interior of the nose, the lining membrane of the 

 cavities of the internal ear, the retina of the eye, &c. ; and 

 as passing towards the central organs, conveying to these the 

 impressions they have received, which impressions may either 

 affect the consciousness, or may excite respondent move- 

 ments, or may act in both modes, through the instrumentality 

 of the vesicular substance to which they are transmitted. 

 The immediate dependence of the functional activity of this 



