PHYSIOLOGY OF THE CRANIAL NEBVES. 533 



to be refracted to a clear image on the retina; and the dilatation in look- 

 ing straight forwards, as in looking at a distant object, permits the ad- 

 mission of the largest number of rays, of which none are too divergent 

 to be so refracted. 



The Fourth Nerve, or Trochlearis. 



Functions. The Fourth nerve, Nervus trochlearis, or Patheticus, 

 is exclusively motor, and supplies only the trochlearis or obliquus supe- 

 rior muscle of the eyeball. It arises from above the fourth ventricle from 

 the valve of Vieussens, but its fibres can be traced to the lower part of 

 the nucleus of the third (Fig. 366) nerve. It decussates with its fellow 

 between its deep and superficial origins. 



The Fifth Nerve, or Trigeminus. 



Functions. The Fifth or Trigeminal nerve resembles, as already 

 stated, the spinal nerves, in that its branches are derived through two 

 roots; namely, the larger or sensory, in connection with which is the 

 Gasserian ganglion, and the smaller or motor root, which has no gan- 

 glion, and which passes under the ganglion of the sensory root to join 

 the third branch or division which ensues from it. The fibres of origin 

 of the fifth nerve appear to come from under the floor of the fourth ven- 

 tricle. The motor root to the inside of the sensory, about the middle of 

 each lateral half. The sensory fibres, however, can be traced down in 

 the medulla as far as the upper part of the cord, these latter fibres bring- 

 ing sensory impressions from the tongue. In addition to these sensory 

 fibres, coming from the nucleus and the spinal cord, there are it is said 

 others coming from the cerebellum. The motor centre is connected with 

 the cerebral cortex of the opposite side. Fibres for the motor root also 

 come from the corpora quadrigemina along the aqueduct of Sylvius. The 

 first and second divisions of the nerve, which arise wholly from the lar- 

 ger root, are purely sensory. The third division being joined, as before 

 said, by the motor root of the nerve, is of course both motor and sen- 

 sory. 



(a.) Motor Functions. Through branches of the lesser or non- 

 ganglionic portion of the fifth, the muscles of mastication, namely, the 

 temporal, masseter, two pterygoid, anterior part of the digastric, and 

 mylo-hyoid, derive their motor nerves. Filaments are also supplied to 

 the tensor tympani and tensor palati. The motor function of these 

 branches is proved by the violent contraction of all the muscles of masti- 

 cation in experimental irritation of the third, or inferior maxillary divi- 

 sion of the nerve; by paralysis of the same muscles, when it is divided 

 or disorganized, or from any reason deprived of power; and by the re- 

 tention of the power of these muscles, when all those supplied by the 



