THE NERVOUS SYSTEM. 195 



in muscles and conducting the stimulus by which the muscle acts, 

 the latter carrying sensation from the periphery to the centre. 



Nerves act as conductors, but do not produce anything. 



A nerve's function is manifested at its terminus. 



Nerve stimulation can be accomplished mechanically, chemi- 

 cally or thermically, also by a mild current of electricity. 



Rate of nervous conduction is 250 feet per second (electrotonus, 

 for lack of space, will not be described). 



The Spinal nerves are 42 or 43 in number, arranged in pairs 

 (for description, see Nerves). The superior or sensory nerves 

 end in the superior or sensory horns of the gray matter of the 

 cord, the inferior or motors end in the inferior or motor horns. 

 Stimulation of the former causes pain at its peripheral distribution, 

 and section loss of sensation at the same point. Stimulation of the 

 latter causes contraction of the muscles to which it is supplied, 

 and section motor paralysis of the same. 



THE CRANIAL NERVES. 



The First or Olfactory will be discussed under Smell. 



The Second or Optic terminates in the retina, the fibres de- 

 cussating. 



Section of one tract before discussation, causes total blindness 

 of the outer half of the eye of the same side and the inner half of 

 the opposite. Injuries of the visual centre produce the same 

 result. 



The Third or Oculo-motor is entirely motor, and supplies all 

 the muscles of the eye, except the external rectus, great oblique 

 and part of the posterior rectus, the pupil with contractility and 

 movements of the ciliary muscles. Section causes external strabis- 

 mus, ptosis, dilation of the pupil, and inability to rotate the globe 

 in a vertical direction. 



The Fourth or Patheticus is entirely motor, and paralysis 

 causes inability to rotate the eye laterally. 



The Fifth or Trifacial is a mixed nerve, viz., both sensory and 

 motor. The ophthalmic and superior maxillary branches are 

 entirely sensory, the inferior maxillary both sensory and motor. 

 Division of the nerve in front of the Gasserian ganglion causes 

 loss of sensation in its course and also trophic changes in the eye- 



