652 THE NERVOUS SYSTEM IN GENERAL. 



At the origin of the trunk into which the nerve roots are collected, 

 there is a greyish enlargement termed a ganglion; but this peculiarity 

 belongs exclusively to the superior fibres. 



After a variable course, which is generally short, this trunk divides into 

 branches, the point of departure for all the nerves of the body. Among 

 these branches, those which are expended in the apparatus of animal life are 

 pairs, and perfectly alike on both sides of the body. Those of the organs 

 of nutrition are composed at first of an almost symmetrical double chain, 

 placed beneath the spinal column, and whose elements are borrowed from 

 nearly all the nervous trunks emanating from the cerebro-spinal axis ; in 

 proceeding to their destination, their distribution is most irregularly com- 

 plicated. As they offer on their course a great number of ganglia similar to 

 those we have already mentioned, they are called ganglionic nerves ; they are 

 also designated the nerves of organic or vegetative life, while the others are 

 named the nerves of animal life or of relation. 



STRUCTURE OP THE NERVOUS SYSTEM. 



Two particular substances, one grey, the other ivhite, enter into the 

 organisation of the nervous apparatus. These two substances are formed, 

 the first by nerve-tubes and united nerve-cells ; the second by tubes alone. 



The nerve-tubes are microscopic elements, composed of a proper wall and 

 contents. The wall, named the nervous sheath (sheath of Schwann), is a thin, 

 homogeneous, elastic membrane that contains in its substance or its inner face 

 some nuclei of cells. It is not visible in quite fresh tubes. The contents 

 comprise, in the centre, a solid core, the axis-cylinder 

 (ox primitive band of Bemak), which becomes very appa- 

 rent after the addition of certain reagents ; between the 

 axis-cylinder and the wall is found a viscid substance, the 

 nervous medulla or white substance of Schivann, which 

 coagulates very quickly under the influence of cold. 



When the medulla is solidified, it is seen to be 

 bordered by two dark lines, parallel to the walls of the 

 nerve-tubes; this aspect has caused the latter to be 

 named " double-contoured tubes (or nerve-fibres)." 

 DIAGRAM OF STRUG- A11 fa e nerv e-tubes do not possess, at the same time, 



TURE OF NERVE .-, ,-, n T, 1 -, 



FIBRK these three parts, lor the medulla may be absent ; so that 



1, Sheath- 2, Medul- there are distinguished medullated and non-medullated 



' lary substance of nerve-fibres. The first, more or less thick, are met with 



Schwann; 3, Axis- in the nervous centres, and at the origin, and in the 



cylinder, or primi- m iddle portion of the nerves ; the second are found at 



the termination of nerves, and in the great sympathetic. 



There are also observed in the nerves of organic life, elongated elements, 



designated fibres of RemaJc, grey fibres, and nucleated nervous fibres. These 



are pale, flattened fibres, with parallel borders, and furnished with elliptical 



nuclei. Some authorities consider these to be bands of connective tissue, and 



not nerve elements. 



The nerve-cells, or corpuscles, are voluminous, and are formed by a mass 

 of granular protoplasm without any enveloping membrane. In the ganglia 

 they are covered by a layer of fibrous connective tissue, provided with nuclei, 

 which appear to furnish them with a very thick enveloping membrane. The 

 nucleus, with one or two nucleoli, is often surrounded by granulations, 

 whose dark colour gives rise to the supposition that they are pigment cells. 



