144 NERVOUS TISSUE, 



Three modes of connection of cells with fibres are described. 1. From 

 a cell, which may have several branched outrunners, one stout unbranched 

 process is continued into a nerve-fibre, at first naked, and probably re- 

 presenting- only the axis-cylinder, then acquiring a medullary sheath 

 and dark borders, and finally a membranous tube or primitive sheath 

 (figs. «9, 90, p. lo-i). 2. From one or more finely divided branches of a 

 cell, or of more than one cell, equally fine fibrils are prolonged, which 

 coalesce into a pale fibre, having the characters of an axis-cylinder, which 

 then, as in the former case, may in its progress become a dark-bor- 

 dered mednllated fibre. 3. The extreme ramifications of a cell or 

 cells, become connected, as in the last case, with fibrils, which join 'into 

 a nerve-filH'e ; but the connection takes place by the intervention of 

 small bipolar cells, which are by one pole continuous with the branches 

 of the larger cell or cells, and by the other with fine fibrils which join 

 into a pale fibre, or into an axis-cylinder of a dark-bordered fibre. 

 Gerlach, and after him Waldeyer and others, have described this last 

 mode of connection, as seen by them in the cerebellum. The statement 

 also derives support from the observations of Lockhart Clarke, on the 

 structure of the olfactory bulb. Along with this indirect connection 

 through small intervening cells, Gerlach supposes that a process or pro- 

 cesses of the large cells pass directly into nerve-fibres ; and should such 

 direct connection take place by the prolongation of an unbranched cell- 

 process into a nerve-fibre, the arrangement would be analogous to that 

 in the ganglia : the simple origin, representing that of the straight 

 fibre from the ganglion-cell, whilst the ramified origin, with the inter- 

 vening small cells, miglit be compared to that of the superficial or spiral 

 fibre, with its interposed nuclei. 



The ijrimitive fibrils may, as already stated, be frequently obsei-ved to pass 

 through the nerve-cells from one process into another. Wherever this is found 

 it seems reasonable to regard the cells rather as interpolations in the course of the 

 nerves than as actually giving origin to them ; this is more especially the case in 

 those instances in which a comparatively small, nucleated swelling on a peripheral 

 fibre represents a nerve-cell. 



The fibres of origin of a nerve, whether deeply implanted or not, on 

 quitting the surface of the brain or spinal cord to form the apparent 

 origin or free part of the root, are in most cases collected into funiculi, 

 which are each invested with a sheath of neurilemma. This investment 

 is generally regarded as a prolongation of the pia mater, and in fact its 

 continuity with that membrane may be seen very plainly at the roots of 

 several of the nerves, es})ecially those of the cervical and dorsal nerves 

 within the vertebi'al canal, for in that situation the neurilemma, like 

 the pia mater itself, is much stronger than in the cranium. The 

 funiculi, approaching each other if originally scattered, advance towards 

 the foramen of the skull or spine which gives issue to the nerve, and pass 

 through the dura mater, either in one bundle and by a single aperture, 

 or in two or more fasciculi, for which there are two or more openings in 

 the membrane. The nerve-roots in their course run beneath the arach- 

 noid membrane, and do not perforate it on issuing from the cranio- 

 vertebral cavity ; for the loose or visceral layer of the arachnoid is pro- 

 longed on the nerve and loosely surrounds it as far as the aperture of 

 egress in the dura mater, where, quitting the nerve, it is reflected upon the 

 inner surface of the latter membrane, and becomes continuous with the 



