PEEIPHEEAL DISTRIBUTION OF NERVES. U5 



parietal or adherent layer of the arachnoid. The nerve, on escaping 

 from the skull or spine, acquires its external, stout, fibrous sheath, 

 which connects all its funiculi into a firm cord, and then, too, the nerve 

 appears much thicker than before its exit. The dura mater accom- 

 panies the nerves through the bony foramina, and becomes continuous 

 with their external sheath and (at the cranial foramina) with the peri- 

 cranium ; but the sheath does not long retain the densely fibrous 

 character of the membrane with which it is thus connected at its com- 

 mencement. 



The an-angement of the membranes on the roots of certain of the cranial nerres 

 requires to be specially noticed. 



The numerous fasciculi of the olfactory nerve pass through their foramina 

 almost immediately after springing from tlie olf actoiy bulb, and then also receive 

 their neurilemma. The bulb itself, and the intracranial 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 fiuTow of the brain in which this part of the nerve lies, without 

 affording it a special investment. 



The optic nerve l^ecomes subdivided internally into longitudinal fascicidi by 

 neuiilemma 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 eyeball. The acoustic nen'e becomes fasciculated, receives its neurilemma, 

 and acquires a finn stn^ctiu-e on entering the meatus auditorius intenius in the 

 temporal bone, towards the bottom of which it presents one or more small gan- 

 glionic swellings containing the characteristic cells. Up to this point it is destitute 

 of neurilemma, and is of soft consistence, whence the name " portio mollis "' 

 applied to it. 



The larger root of the fifth pair acquires its neuiilemma and its fasciculated 

 character sooner at its cii'cumference 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 de- 

 scribed elsewhere in the special anatomy of these nerves. 



Termixatiox, oe Peripheral Distributiox, of Nerves.— It may 

 be stated, generally, and apart from what may apply to special modes 

 of termination, that, in approaching their final distribution, the _/2hrcs of 

 nerves, medullated and non-meduHated, commonly divide into branches 

 (fig. 98) ; 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 membranous tube continues some way along 

 these pale branches after the medullary sheath has ceased, but may 

 finally too desert them. By repeated division the fibres become smaller 

 and smaller ; but whilst some of the resulting small^ fibres may be 

 simple, many are really bundles of exquisitely fine pale fibrils, straight, 



