SPINAL MARROW. 1-i? 



preserve their form. Besides these, there is a delicate cellu- 

 lar membrane, containing bloodvessels, which penetrates the 

 interior, and forms sheaths round the component filaments. 

 This membrane is termed Neurikma. It may be exhibited 

 by dissolving the substance of the nerve in caustic potash ; 

 or the existence of filaments may be shewn by removing the 

 neurilema by acids. In this last case, they may be observed 

 anastamosing in various ways. 



Some of the nerves appear to have a simple origin ; but 

 in general, several filaments, from different parts of the 

 brain or spinal marrow, unite to form the trunk of a nerve. 

 This trunk again subdivides in various ways ; but the ra- 

 mifications do not always exhibit a proportional decrease of 

 size. It frequently happens, that the branches of the same 

 or of different nerves, unite and separate repeatedly within 

 a small space, forming a close kind of net- work, to which the 

 name Plexus has been applied. Sometimes filaments pass 

 from one nerve to another; and, at the junction, there is 

 usually an enlargement of medullary matter termed a Gan- 

 glion. Numerous filaments, from different nerves, often 

 unite to form a ganglion, from which proceed trunks fre- 

 quently of greater magnitude than the filaments which en- 

 tered. Thus nerves, very different in their origin, form 

 communications with one another ; so that the whole ner- 

 vous system, may be considered as a kind of net-work, be- 

 tween the different parts of which, an intimate connection 

 subsists. In consequence of this arrangement, it .is often mat- 

 ter of very great difficulty to ascertain the origin of those 

 filaments which unite to constitute the trunk of a nerve. In 

 some instances, they appear to arise from the surface of the 

 brain or spinal marrow ; in other cases, from the more cen- 

 tral parts. 



The manner in which the nerves terminate in the diffe- 

 rent organs to which they are distributed, is still consi- 



K 2 



