330 NERVOUS SYSTEM. 



the body, by means, in all probability, of direct continuity of fibre. 

 If one of the lateral halves of the spinal marrow be cut through, 

 the animal will become paralytic, not on the opposite, but on the 

 same side ; a fact which, although it at first view appears con- 

 tradictory to what we have just stated, in truth tends to confirm 

 this opinion : for the medulla spinalis being composed of lateral 

 portions, the fibres decussate each other in the same manner 

 as those of the nerves are supposed to do. 



Ramljication. — The nerves of motion, before entering the mus- 

 cles, form various communications with others in the vicinity ; 

 in many parts, by such frequent intercourse, that a kind of ner- 

 vous network is formed, to which the term plexus is applied. 

 And the plexus is intricate in proportion to the number of mus- 

 cles to be supplied, and the variety of combinations into which 

 the muscles enter*. In their course, the nerves generally 

 proceed in straight lines to the parts to which they are dis- 

 tributed ; deviating only (like the arteries) for their own safety, 

 or for some wise and evident purposef . Sometimes they run 

 with the bloodvessels, sometimes alone : we commonly find 

 a nervous trunk, and in some places two, accompanying the 

 principal arteries and veins of the extremities. The Joranches 

 of the nerves, for the most part, come off at acute angles : 

 those springing immediately from the trunk send off others 

 of smaller size, until filaments of such minuteness are formed, 

 as to be invisible to the naked eye. 



Termination. — The twig-like ramifications of nerves end in two 

 different ways : either by inter-communication — which is similar 

 to the anastomosis of arteries, or by sentient extremities within 

 the substance of those organs to which they are distributed. In 

 the retina (a part of the eye entirely composed of the expanded 

 termination of the optic nerve) an extremely delicate tissue, of a 

 pulpy consistence, and semipellucid yellowish appearance, is 

 observable ; from which it has been conjectured, that the extre- 

 mities of other nerves may be somewhat similar: but, to confess 

 the truth, we do not know what form they actually assume — we 

 think it very probable, that their mode of termination may vary 

 according to the nature and texture of the part in which they arc 



* The Nervous System. By C. Bell, F.R.S. 



f There is no such thing as irregularity in the nervous system. The 

 term may be applicable to arteries or veins, because it si;^nifies not whether 

 a part be supplied with this or with that branch, so that arterial I)loofl is 

 furnished. But one nerve cannot supply the office of another. (The pro- 

 perty dispended resulting from the source from which the nerve is derived). 

 There is no such thing as a nerve deviating or being found wanting (an 

 occurrence frequent in the vascular system) without the loss of some essen- 

 tial i^cuMy.— C.Bell, F.R.S. 



