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 jplexus 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 
