THE NERVES OF SENSATION, 



387 



sentinels, from countless injuries. For how many dreadful 

 wounds would be constantly ensuing if the sensation of pain 

 communicated by the cutaneous nerves to the brain did not 

 warn us to be careful, or teach us by repeated experiences to 

 guard our movements ! 



Thus the senses connect us in a truly marvellous manner with 

 the various phenomena of the external world ; but the brain is 

 meant to be not merely a recipient of sensations, but also to impel 

 our limbs to action; and. this highly important function is 

 likewise performed through the agency of peculiar nerves, which 

 partly emerge from various small orifices at the basis of the 

 skull, partly run through the spinal cord whose anterior columns 

 they form. From these their fibres emerge, in a similar manner 

 with the sensitive filaments, in thirty- 

 one pairs, which, soon after their emis- 

 sion, commingle with the corresponding 

 sensitive fibres, and form a compound 

 nerve, which distributes its branches 

 to the muscles, or to the integuments 

 of a part of the body or the limbs. 

 Thus each spinal nerve has two roots, a 

 sensitive and a motile one, and each 

 root is composed of a large number of 

 independent fibres. The sensitive fila- 

 ments, destined to lead impressions to 

 the brain, and utterly incapable of exe- 

 cuting its orders, chiefly terminate in 

 the skin; while the motile filaments, 

 destined to transmit commands from the 

 brain, and utterly insensible, terminate 

 exclusively in the muscles, which at their 

 bidding contract or relax their hold. 



Thus each nervous filament has its 

 peculiar task to perform, and the salu- 

 tary effects of the division of labour 

 are nowhere more apparent than in the economy of our body. 

 What a dreadful confusion if the optic nerve, for instance, 

 had to be the mediator of feeling, or if the motile nerves 

 had to perform the functions of smelling or tasting ! 



c c 2 



Origin of a Spinal Nerve. 

 (Magnified.) 



A. Anterior root; P. posterior root ; 

 G. ganglion on the posterior roct ; 

 C. compound nerve resulting from 

 the commingling of the fibres 

 both roots. 



of 



