328 NERVOUS SYSTEM. 



OF THE NERVES. 



The nerves are dense, white, fibrous chords, proceeding from 

 the brain and spinal marrow to all parts of the body. 



Number. — From the brain issue ten pairs, denominated cere- 

 bral nerves ; from the spinal marrow, thirty-six, termed spinal : 

 making, altogether,yor(y-su' pairs*. 



Magnitude. — In some animals (among which may be num- 

 bered the horse) the nerves, taking them generally, are of larger 

 size than they are in the human body, although the brain of 

 the latter far preponderates in volume. According to Richerand, 

 " the spinal marrow and the nerves, in the different animals 

 furnished with them, are larger in proportion to the brain, ac- 

 cording as the animal is more distant from man in the scale of 

 animation." 



Coverings. — The nerves are covered by the same membranes 

 as cover the brain : the one may be traced from the other. The 

 pia mater, indeed, enters into the composition of the nerve, form- 

 ing minute sheaths in which the nervous matter is contained. 

 These external coverings, however, seem to proceed only to a short 

 distance ; for if we examine the outer cases in which they 

 are subsequently inclosed, we shall find them to be nothing 

 more than condensed celkdar membrane. It is to the external 

 covering that the compactness and density of a nerve are entirely 

 owing; when deprived of it, but a slight degree of pressure will 

 destroy its texture : indeed, there are some nerves whose exposed 

 situation would subject them to perpetual contusion and lacera- 

 tion, were it not for the protection afforded them by this compact 

 cellular envelope. 



Structure. — The substance of the nerves is pulpy ; and their 

 pulpy matter is contained within minute membranous sheaths or 

 tubes (composed either of simple cellular membrane or of pia 

 mater), constituting in this manner so many filaments or funiculi, 

 which form one continuous tract from their exit from the brain to 

 their ending in the skin or muscle. Every one of these filaments 



* " In the view which I have taken of the nerves of the (human) body, 

 there are, besides the nerves of vision, smell, and hearinaf, four systems 

 combined into a whole. Nerves entirely different in function extend 

 through the frame : 1 st, those of sensation ; 2dly, those of voluntary motion ; 

 3dly, those of respirator!/ motion ; and, lastly, nerves constituting the st/mpa- 

 thetic si/stem, which froin their being- deficient in qualities that distinguish 

 the three others, seem to unite the body into a whole, in the performance of 

 the functions of nutrition, growth, and decay, and whatever is directly ne- 

 cessary to animal existence. Of these, the two first are bound together 

 through their whole course ; the third are partially joined to the two former; 

 and the last are the most irregular of all." — Such is a coup d'crit of the 

 new arrangement of the nervous system introduced by Sir Charles Bell. 



