REPORT ON THE PHYSIOLOGY OF THE NERVOUS SYSTEM. 81 



light on the functions of several of these nerves. The motions of 

 the iris, he shows, require the integrity of the third pair, division 

 of these nerves being always followed by full dilatation of the 

 pupils, which cease to be obedient to the stimulus of light. If the 

 divided end of the nerve communicating with the eye be pinched 

 by the forceps, the iris contracts. Hence it is apparent that dimi- 

 nution of the aperture of the pupil is the result of action, and 

 dilatation of the pupil the result of relaxation, of the iris. Flou- 

 rens has shown that complete extirpation of the tubercula quad- 

 rigemina also paralyses the iris, and that irritation of those bo- 

 dies excites its contractions. The same effect is noticed by Mayo 

 to arise from division or irritation of the optic nerve. He divided 

 the optic nerves within the cranium of a pigeon immediately 

 after decapitation. When the end of the nerve connected with 

 the ball of the eye was seized in the forceps, no action ensued; 

 but when the end attached to the brain was irritated, the iris 

 immediately contracted. These several experiments clearly in- 

 dicate the dependence of the iris upon the optic nerve, upon 

 the tubercula from which one root of that nerve springs, and 

 upon the third pair. The stimulus of light impinges upon the 

 retina, is conveyed along the optic nerve through the tubercle 

 to the sensorivmi, whence the motive impression is propagated 

 to the iris by the third encephalic nerve. 



It is not so easy to define the precise mode of action of the 

 pathetici, or fourth pair of nerves. Sir Charles Bell * supposes 

 that they are destined " to provide for the insensible and in- 

 stinctive rolling of the eyeball, and to associate this motion of 

 the eyeball with the winking motions of the eyelids." He even 

 conjectures that "the influence of the fourth nerve is, on cer- 

 tain occasions, to cause a relaxation of the muscle to which it 

 goes." It is certain, however, from its exclusive distribution to 

 the superior oblique muscle, that the fourth is a nerve of motion. 

 The sixth nerve is also a nerve of voluntary motion, and is sent 

 to the rectus externus of the eyeball. 



Sir Charles Bell has placed the portio dura of the seventh 

 pair among his respiratory nerves. There is, however, no doubt 

 that it is simply a motive nerve, and that it is indeed the only 

 nerve of motion, which supplies all the muscles of the face, 

 except those of the lower jaw and palate. Division of this 

 nerve occasions no expression of pain, according to Bell; but 

 Mayo's experience is opposed to this absence of sensibility f. 

 "The motion of the nostril of the same side instantly ceased, 



* Natural System of Nerves, p. 3.58. 



t See Mr. H. Mayo's Anatomical and Pliijsioloffical Commentaries, Part I.; 

 and Outlines of Human PLy^ioloijij, 2nd edit., p. 3;34, 

 1 S.Jo. G 



