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range of influence, and in mode of action. The cerebro-spinal system 
includes the brain and spinal cord, and the various nerves proceeding from 
them. ,The sympathetic system consists of a double chain of nerves—one 
on each side of the backbone—from which branches are distributed. The 
nervous apparatus is made up of two ultimate factors, nerve-fibres and 
nerve-cells, and these are intimately associated together. The cells are 
collected together in groups or masses, and are always mingled more or less 
with fibres, and both together form what is.termed a “nerve-centre.” The 
fibres, besides entering into the composition of nerve-centres, form nerves, 
which connect the different centres, and are distributed to the various parts 
of the individual. Nerve-cells and nerve-fibres differ in function. The 
former generate and conduct nerve force, while the latter merely conduct it. 
‘We may compare the nervous system with a galvanic battery, and the 
telegraphic wires proceeding from it. The battery, like the nerve centre, 
generates and conducts the current; while the wires, like the nerves, merely 
conduct it, having no share whatever in its production. 
Nervous force travels at a very quick rate. It has been calculated 
by physiologists, that the rate of conduction in human nerves supplying 
muscles with motor power is 111 feet per second, and that in those nerves 
by which sensation is conducted, it is still quicker, reaching as high as 140 
feet per second. Each nerve is composed of a variable number of bundles 
of nerve-fibres, which have separate sheaths. The bundles of fibres, also, 
have separate sheaths, and the whole of them in turn are enclosed in a firm 
fibrous covering. 
—_ 1 
—— 
we 
x 
& 
Figure A shows a nerve fibre (after Klein) magnified 300 times. 1 is 
the sheath, 2 is the medulla, and 3 is the axis-cylinder. The constriction in 
the centre, where the medulla is deficient, is called a node of Ranvier. 
Figure B shows a bundle of nerve-fibrils cut transversely, and parts of two 
others. Several such bundles make up a nerve. This specimen is from the 
nerve of a dog, highly magnified. 
