95 



range of influence, and in mode of action. The cerebro-spinal system 

 includes the brain and spinal cord, and the \arious ner\-es proceeding from 

 them. The sympathetic system consists of a double chain of nerves— one 

 .on each side of the backbone — from Avhich branches are distributed. The 

 Tier\'0us 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 '' ner\-e-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 

 fomier 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 batteiy, like the nerve centre, 

 generates and conducts the current ; while the wires, like the ner\'es, merely 

 conduct it, having no share whatever in its production. 



Nervous force travels at a ver>^ quick rate. It has been calculated 

 by physiologists, that the rate of conduction in human nerves supplying 

 muscles with motor power is 1 1 1 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. 



, J 



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Y' 



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Figure A shows a ner\^e fibre (after Klein) magnified 300 times, i is 

 the sheath, 2 is the medulla, and 3 is the axis-cylinder. Th# 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 ncnc. This specimen is from the 

 nen-e of a dog, highly magnified. 



