1116 



GENERAL ANATOMY OR HISTOLOGY. 



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

 also the greater part of the cerebro-spinal nerves, and gives to these struct- 

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

 geneous ; but soon after removal from the body they present, when examined by 

 transmitted light, a double outline or contour, as if consisting of two parts 

 (Fig. 652). The central portion is named the axis-cylinder of Purkinje ; around 

 this is a sort of sheath of fatty material, staining black with osmic acid, named 



FIG. 652. White or medullated nerve- 

 fibres showing the sinuous outline and 

 double contours. (After Schafer.) 



_ _ Fibrils of axis- 

 cylinder. 



Neurilemma. 



Segment of 

 Lantermann. 



FIG. 653. Longitudinal section through a nerve-fibre from 

 the sciatic nerve of a frog. X 830. (After Bohm and. 

 Davidoff.) 



the white substance of Schwann, which gives to the fibre its double contour, and 

 the whole is enclosed in a delicate membrane, the neurilemma^ primitive sheath, 

 or nucleated sheath of Schwann (Fig. 652). 



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 occa- 

 sionally 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 

 constitutes about one-half or one-third of the nerve-tube, the whole substance being 



greater in proportion in the 

 nerves than in the central organs. 

 It is perfectly transparent, and 

 is therefore indistinguishable in 

 a perfectly fresh and natural 

 state of the nerve. It is made 

 up of exceedingly fine fibrils, 

 which stain darkly with gold 

 chloride (Fig. 653). At its 

 termination the axis-cylinder of a nerve-fibre may be seen to break up into fibrillae, 

 confirming the view of its structure. These fibrillae have been termed the primi- 

 tive fibrillce of Schultze. The axis-cylinder is said by some to be enveloped in a 

 special, reticular sheath, Avhich separates it from the white matter of Schwann, and 

 is composed of a substance called neurokeratin. The more common opinion is that 

 this network or reticulum is contained in the white matter of Schwann, and by 

 some it is believed to be produced by the action of the reagents employed to show 

 it. The medullary sheath or white matter of Schwann (Fig. 653) 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 



FIG. 654. 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- 

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

 the sheath of Schwaun appears thickened. (Klein and Noble 

 Smith.) 



