316 NERVOUS SYSTEM. 







sufficiently represented by the plan of the large nervous plex- 

 uses, as they occur in various parts of the body. It is the ge- 

 neral envelope which is obviously continuous with the pia ma- 

 ter, but the particular sheaths of the finer fibres are lost insen- 

 sibly, so that these fibres appear naked in the centre of the 

 nerve, at its central extremity. The same destitution of neu- 

 rilematic covering is observable at the peripheral extremities of 

 the nerves, wherever the laUer can be traced. The interior of 

 these canals is traversed by processes, which cross the nervous 

 matter and sustain it. From the increase in size, the additional 

 solidity, and the close adhesion of the nerves to the dura ma- 

 ter, where they pass out of their several foramina in the spine 

 and cranium, there is no doubt that the dura mater contributes 

 to the neurileme, though its structure is altered and made much 

 less dense. The best evidence of this is the sheath of the optic 

 nerve, and of the spinal nerves. This opinion, advanced by the 

 ancients, has been strongly contested by Haller,* and by Zinn.f 

 The tunica arachnoidea is too fine to admit of any positive opi- 

 nion about the extent to which it follows the nerves. 



The neurileme has but little contractility, is solid and diffi- 

 cult to tear, and is supposed to be the secretory organ of the 

 medullary substance. 



The nervous fasciculi are, moreover, held together by cellu- 

 lar substance, which has in the progress of life, a tendency to 

 the deposite of fat. This cellular substance, in neuralgic affec- 

 tions, is subject to infiltrations and redness, whereby it becomes 

 hard. This circumstance has induced pathologists to consider 

 the pain as depending upon its inflammation. J 



The optic nerve, owing to the size of its canals, furnishes the 

 best example of structure, and the nerves of the muscles are 

 next. There are, however, some peculiarities in different 

 nerves; as the observations of Sir Everard Home have ascer- 

 tained that the medullary filaments of the optic nerve augment 

 in numbers and diminish in volume, from its origin towards its 

 termination. The principal light thrown upon these minute 

 and interesting points of nervous organization, has been de- 

 rived from the researches of Reil. 



* Prim. Lin. t Memoires de Berlin, 1753. 



\ Beclard, Anat. Gen. p. 665. 



Reil, de Structura Nervorum. Halte Saxonum, 1796. 



