STRUCTURE OF THE NERVE FIBRES. 149 



varicose enlargements, having a more or less regular fusiform 

 shape, and these subsequently, by further imbibition, increase 

 in size and number until at length the fibre becomes unrecog- 

 nisable and disappears. 



A second kind of fibre very commonly met with in the 

 central organs, is distinguished from the foregoing by its 

 greater thickness. Such fibres are very delicate, transparent, 

 and perishable, of albuminous composition, and only isolable 

 for short tracts. Their diameter is very various, amounting 

 only to a few micromillimeters. Speaking generally, they are 

 the fibres which have been termed naked axis cylinders. 

 The thicker they are the more easy is it to distinguish their 

 internal structure. This presents a more or less well-marked 

 longitudinal striation, resulting from a fibrous differentiation 

 of the substance of the fibre, and from the presence in all 

 probability of an interfibrillar finely granular material. It 

 is most easy to discern that they are composed of fibrils in 

 the thick-branched processes of the large centric ganglion cells, 

 which Deiters proposed to call protoplasmic processes, a name 

 for which I substituted the term ramifying processes* More- 

 over, the axis-cylinder processes even of these ganglion cells and 

 other fibres of the central organs of the nervous system usually 

 regarded as naked cylinders, and believed to run for consider- 

 able distances without branching, often present a distinctly 

 fibrillar structure. Their fibrillar character is most distinctly 

 visible at their origin from the ganglion cells, as shown in fig. 18 

 (at xx). I have applied the term " primitive-fibril bundles, or 

 fasciculi," to this second kind of fibre. 



Both kinds of fibres, the primitive fibrils and the fibrillar 

 fasciculi, may become invested with a medullary sheath, as in 

 the adjoining figure (at a), and thus become converted into a 

 third form of nerve fibre, the medullated. The medullated 

 fibres therefore consist essentially of two constituents, a cortex 

 or sheath of medullary nerve substance, and an axial fibre or 

 axis cylinder, which is either a primitive fibre or a bundle of 

 fibrillse. The medullary sheath forms a more or less thick 



* See Deiters' Researches on the Brain and Spinal Cord, Brunswick, 

 1865 ; and my Preface to his Book, pp. xv. xvii. 



