TERMINATION OF NERVES. cxlix 



but the sheath does not long retain the densely fibrous character of the 

 membrane with which it is thus connected at its commencement. 



The arrangement of the membranes on the roots of certain of the cranial nerves 

 requires to be specially noticed. 



The numerous fasciculi of the olfactory nerve pass through their foramina almost 

 immediately after springing from the olfactory bulb, and then also receive their 

 neurilemma. The bulb itself, and the intracrauial part of the nerve, which are to be 

 regarded as being really a prolongation or lobe of the brain, are invested externally 

 by the pia mater, but are not fasciculated. The arachnoid membrane passes over 

 the furrow of the brain in which this part of the nerve lies, without affording it a 

 special investment. 



The optic nerve becomes subdivided internally into longitudinal fasciculi by neu- 

 rilemma a little way in front of the commissure : on passing through the optic 

 foramen it receives a sheath of dura mater, which accompanies it as far as the eye- 

 ball. The acoustic nerve becomes fasciculated, receives its neurilemma, and acquires 

 a firm structure on entering the meatus auditorius internus in the temporal bone, 

 towards the bottom of which it presents one or more small ganglionic swellings con- 

 taining the characteristic cells. Up to this point it is destitute of neurilemma, and 

 of soft consistence, whence the name " portio mollis " applied to it. 



The larger root of the fifth pair acquires its neurilemma and its fasciculated cha- 

 racter sooner at its circumference than in the centre, so that, in the round bunch of 

 cords of which it consists, those placed more outwardly are longer than those within, 

 and, when all are pulled away, the non-fascicular part of the nerve remains in form of 

 a small conical eminence of comparatively soft nervous substance. 



Most of the nerves have ganglia connected with their roots. Thus, the 

 spinal nerves have each a ganglion on the posterior of the two roots by 

 which they arise ; and in like manner several of the cranial, viz., the fifth, 

 seventh, glosso-pharyngeal, and pneumo-gastric, are furnished at their 

 roots, or at least within a short distance of their origin, with ganglia which 

 involve a greater or less number of their fibres, as described elsewhere in 

 the special anatomy of those nerves. 



Termination, or peripheral distribution, of nerves. It may be stated, 

 generally, and apart from what may apply to special modes of termination, 

 that, in approaching their final distribution, ihe fibres of nerves, medullated 

 and non-medullated, commonly divide into branches (fig. LXXXV) ; and the 

 former, either before or after division, generally lose their medullary 

 sheath, and consequently their dark borders, and take on the characters of 

 pale fibres. The axis-cylinder participates in the division, and it might be 

 said that the white fibres are represented in their further progress by the 

 axis-cylinder and its ramifications ; still, the primitive sheath or mem- 

 branous tube continues some way along these pale branches after the medul- 

 lary sheath has ceased, but may finally too desert them. By repeated division 

 the fibres become smaller arid smaller ; but whilst some of the resulting small 

 fibres may be simple, many are really bundles of exquisitely fine pale fibrils, 

 straight, sinuous, or somewhat tortuous in their course. They bear nuclei, 

 some of which, no doubt, may appertain to the prolongation of the primitive 

 sheath ; but others, generally fusiform and granular, are interposed, as it were, 

 in the course of the fibres, and are continuous with them at either end ; 

 nuclei, moreover, of a triangular or irregular shape, are common at the 

 bifurcations of the fibres. These pale fibres often join into networks ; but 

 their further disposition in different parts will be treated of below. In 

 the meantime it must be explained that the original dark-bordered fibres 

 which thus undergo division and change, or which may proceed singly to 

 end in a different and special manner, are commonly provided with a 

 tolerably strong sheath with nuclei, which, as it stands well apart from 



