92 COMPARATIVE ANATOMY 



is pear-shaped and enclosed in a distinct sheath. From its 

 narrower end two processes are given off, of which one, the 

 larger, starts from the deeper part of the body of the cell 

 and pursues a straight course, being, in fact, the direct con- 

 tinuation of a nerve -fibre. The second smaller fibre starts 

 from a plexus of fine fibrillae on the surface of the cell-body, 

 and is wound spirally round the larger fibre, eventually quit- 

 ting it and taking an opposite course. 



Nerve-fibres are of two kinds, white or medullated nerve- 

 fibres, and grey or non-medullated nerve-fibres. The latter 

 are chiefly found in the sympathetic system, but also occur, 

 though not abundantly, in the cerebro-spinal nerves. They 

 are transparent fibres with a faint longitudinal striation, and 

 exhibit nuclei at frequent intervals on their courses. It is 

 doubtful whether the nuclei lie outside the fibre, enclosed in 

 a delicate investing sheath, or whether they are embedded in 

 the superficial layer of the fibre itself. 



White medullated nerve-fibres form the bulk of the white 

 matter of the brain and spinal cord, and of the cerebro-spinal 

 nerves. They are called " white " because, shortly after death, 

 a peculiar fatty substance forming their sheaths sets hard and 

 white, but during life it is liquid, and the fibres have then a 

 pale transparent aspect. A single white fibre may be roughly 

 compared to an insulated telegraph wire. The transmitting 

 part, the actual wire, is represented by a pale strand of tissue, 

 having the characters of a non-medullated fibre, and known 

 as the axial fibre or axis-cylinder. It is nearly certain that 

 every axis-cylinder is a direct prolongation of a process of a 

 nerve ganglion cell, and it is to be remarked that the axis- 

 cylinder of a nerve fibre runs an uninterrupted course from 

 the nerve-cell from which it originates to its peripheral ter- 

 mination. The insulating gutta-percha sheath of a telegraph 

 wire is represented in the nerve fibre by a sheath of a medul- 

 lary substance, or myelin, composed chiefly of the peculiar 

 phosphorised fat, lecithin, and the calico or canvas outer 

 wrapping of the telegraph wire is represented in the nerve- 

 fibre by a delicate external homogeneous covering, called the 

 primitive sheath of Schwann, or the neurilemma. 



The neurilemma forms a continuous covering to the nerve- 

 fibre, but the medullary substance is broken up into segments 

 by constrictions, placed at regular intervals, and known as the 



