740 



TEE NERVOUS SYSTEM IN GENERAL. 



407. 



riAGRAM OF STRUC- 

 TURE OF NERVE- 

 FIBRE. 



1, Sheath; 2, medul- 

 lary substance of 

 Schwann ; 3, axis- 

 cylinder, or primi- 

 tive band. 



Fig. 408. 



brane, within wliich is a layer of protoplasm containing a nucleus. The axis of 

 the segment is occupied by a slender cylindrical stalk {axis-cylinder of Fiirkinje, 

 or band of'Remak), the space between it and the protoplasm 

 being filled by a viscid opaque substance {myelin, medtdht, 

 white substance of Schwann, or sheath of the marrovf), whi(;h 

 i8 stained black by osmic acid, and is quickly coagulated 

 by cold. 



The axis-cylinder is not interrupted at the points of con- 

 tact of the segments, but passes through the annular con- 

 strictions, and runs from beginning to end of the nerve. 



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-con- 

 toured tubes (or nerve-fibres)." 



All the nerve-tubes do not possess, at the same time, 

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

 there are distinguished medullated and non-medullated nerve- 

 fibres. The first, more or less thick, are met with in the nerve-centres, and at 

 the origin and middle portion of the nerves ; the second are found at the termi- 

 nation of nerves, and in the great sympathetic. 



There are also observed in the nerves of organic 

 life, elongated elements, designated ^ftres of Renmk,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 

 formed by a mass of protoplasm without any enveloping 

 membrane, but in the midst of which are numerous 

 fibrillar. In the ganglia they are covered by a layer of 

 fibrillar 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 granules of a brown colour. 



The nerve-cells have prolongations or poles, the 

 number of which varies from one to five. Cells with 

 only one prolongation are named unipolar ; those which 

 have two are bipolar ; and those which have a greater 

 number are designated multipolar. In Mammalia, one 

 of these prolongations — nerve prolongation, prolongation 

 of Deiters — in the spinal cord establishes relations be- 

 tween a cell and nerve-tube ; the others are of a proto- 

 plasmic nature, and ramify — these ramifications joining 

 those of neighbouring cells. 



Such are the anatomical elements that enter into the 

 structure of the nervous system. 



In the white substance of the cerebro-spinal axis, 

 only medullated nerve-tubes of every size are found ; in 

 the grey substance are tubes, and a more or less con- 

 siderable number of ntsrve-cells are situated along their course. 



NERVE-TUBE. 



X, Nerve-tube slightly mag- 

 nified : a, annular con- 

 striction ; h, nucleus of 

 the interannular segment ; 

 c, axis-cylinder. B, An- 

 nular constriction and por- 

 tions of the interannular 

 segments highly magnified 

 (prepared with osmic acid, 

 which colours the myelin 

 black) : a', annular con- 

 striction, or node of Ran- 

 vier ; 6', nucleus of the 

 interannular segment; c', 

 external nucleus of the 

 sheath. 



