NERVOUS TISSUE. 



71 



nervous cords, and also constitutes a great part of the brain and spinal cord. 

 The fibres of which it consists are of two kinds, the medullated or white fibres, 

 and the non-medullated or gray fibres. 



The medullated fibres form the white part of the brain and spinal cord, and 

 also the greater part of the cerebro-spinal nerves, and give to these structures 

 their opaque, white aspect. When perfectly fresh they appear to be homo- 

 geneous ; but soon after removal from the body they present, when exam- 

 ined by transmitted light, a double outline or con- 

 tour. as if consisting of two parts. The central 

 portion is named the axis-cylinder of Purkinje ; 

 around this is a sort of sheath of fatty material, 

 named the white substance of Sch ivann, Avhich gives 

 to the fibre its double contour, and the whole is en- 

 closed in a delicate membrane, the neurilemma, 1 

 primitive sheath, or nucleated sheath of Schwann 

 (Fig. 47 



The axis-cylinder is the essential part of the 

 nerve-fibre, and is always present ; the other parts, 

 the medullary sheath and the neurilemma. being 

 occasionally absent, especially at the origin and 

 termination of the nerve-fibre. It undergoes no 

 interruption from its origin in the nerve-centre to 

 its peripheral termination, and must be regarded 

 as a direct prolongation of a nerve-cell. It con- 

 stitutes about one-half or one-third of the nerve- 

 fibre, the white substance being greater in propor- 

 tion in the nerves than in the central organs. It 

 is perfectly transparent, and is therefore indistin- 

 guishable in a perfectly fresh and natural state of 

 the nerve. When examined under a high power it 

 presents the appearance of longitudinal striation. as 

 if composed of very fine, homogeneous fibrilla?, held together in a faintly granular 

 interstitial material. Occasionally at its termination the axis-cylinder of a fibre 

 may be seen to break up into exceedingly fine fibrillae, confirming the view of its 

 fibrillar structure. These fibrillse have been termed the primitive fibrillce of 

 Schultze. The axis-cylinder is said to be enveloped in a very delicate, hyaline 

 sheath, which separates it from the white matter of Schwann. The medullary 

 sheath or white matter of Schwann is regarded as being a fatty matter in a fluid 

 state, which insulates and protects the essential part of the nerve the axis-cylinder. 

 The white matter varies in thickness to a very considerable extent, in some forming 

 a layer of extreme thinness, so as to be scarcely distinguishable, in others forming 

 about one-half the nerve-tube. The size of the nerve-fibres, which varies from 

 TT5T to -> 0*0 f an i ncn > depends mainly upon the amount of the white substance. 

 though the axis-cylinder also varies in size Avithin certain limits. The white matter 

 of Schwann does not always form a continuous sheath to the axis-cylinder, but 

 undergoes interruptions in its continuity at regular intervals, giving to the fibre 

 the appearance of constriction at these points. These were first described by Ran- 

 vier. and are known as the nodes 

 of Ranvier (Fig. 48). The por- 

 tion of nerve-fibre between two 

 nodes is called an internodal seg- 



t. The neurilemma or prim- 



itivp ssliparh i* tint intprrnntpfl at 



1 dr 



the nodes, but passes over them 



-r, i 



as a continuous membrane, .hacn 



1 In older hbtological works the term " neurilemma " is used to designate the fibrous envelope of 

 the whole nerve, now called " perineuriuru.'' 



FIG. 47. Human nerve-fibres. Mag- 

 nified 350 times. Three of them are fine, 

 one of which is varicose, one of mid- 

 dling thickness, and with a simple con- 

 tour : and three thick, two of which are 

 double-contoured, and one with gru- 

 mous contents. 



Fio.48.-A node of Ranvier of a medullated nerve-fibre, 

 viewed from above, magnified about 750 diameters. The medul- 

 lary sheath is discontinuous at the node, whereas the axis-cyl- 

 inder passes from one segment into the other. At the node the 

 sheath of Schwann appears thickened. (Klein and Noble 



