154 FUNCTIONS OF THE NERVOUS SYSTEM. 



loathing. There is another protective influence exerted by it, of a still more 

 remarkable nature. It has been ascertained by Dr. M. Hall that, if the func- 

 tions of the Brain be suspended or destroyed, without injury to the Spinal 

 system of nerves, the Orbicularis muscle will contract, so as to occasion the 

 closure of the eyelids, upon the tarsal margin being touched with a feather. 

 This fact is interesting in several points of view. In the first place, it is a 

 characteristic example of pure reflex action, occurring under circumstances in 

 which volition cannot be imagined to guide it, and in which there is no valid 

 reason to believe that sensation directs it. Further, it explains the almost irre- 

 sistible nature of the tendency to winking, which is performed at short inter- 

 vals by the contraction of the Orbicularis muscle ; this is evidently a Spinal 

 action, capable of being in some degree restrained (like that of respiration) by 

 the will, but only until such time as the stimulus (resulting, perhaps, from the 

 collection of minute particles of dust upon the eyes, or from the dryness of its 

 surface in consequence of evaporation), becomes too strong to be any longer 

 resisted. Again, we have in sleep or in apoplexy an example of this purely 

 spinal action, unbalanced by the influence of the will, which, in the waking 

 state, antagonizes it by calling the levator palpebrae into action. As soon as 

 the will ceases to act, the lids droop, and close over the eye in order to protect 

 it ; and if those of a sleeping person be separated by the hand, they will be 

 found presently to return. Here, as in studying the respiratory and other 

 movements, we are led to perceive, that it is the Brain alone which is torpid 

 during sleep, and whose functions are affected by this torpidity. As Dr. M. 

 Hall very justly remarks, the Spinal system never sleeps ; it is constantly in 

 activity ; and it is thus that, in all periods and phases of life, the movements 

 which are essential to its continued maintenance are kept up without sensible 

 effort. 



205. The closure of the Pupil against a strong light is another movement 

 of the same protective tendency. The channel through which that just 

 named is performed, is completed by the first branch of the Fifth and the 

 Portio Dura of the seventh. The contraction of the pupil is immediately 

 caused by the Third pair, or Motor Oculi, as is easily shown by irritating 

 the trunk of that. nerve and observing the result. But it is not easy to speak 

 with certainty as to the afferent nerve, by which the motor influence is excited. 

 Although the contraction of the pupil is usually in close accordance with the 

 sensation occasioned by the impression of light upon the retina, yet there is 

 no want of evidence to prove that the sensation of light is not always necessary ; 

 for, even when the sight of both eyes has been entirely destroyed by amau- 

 rosis, the regular actions have been witnessed in the pupil, in accordance 

 with varying degrees of light impinging on the retina. This fact may be 

 explained in two ways. It may either be imagined that the requisite stimu- 

 lus is not that of light conveyed through the optic nerve, but that of heat 

 conveyed through the ophthalmic branch of the Fifth pair. Or it may be 

 still supposed, that the motion results from an impression upon the retina, 

 which impression, being conducted to the Brain, ordinarily produces a sensa- 

 tion ; whilst in these curious cases, no sensation is produced, on account of a 

 disordered state of the part of the Brain, in which the Optic nerve terminates ; 

 though some filaments of that nerve, being connected with the Spinal Cord, 

 and not with the Brain, can produce a reflex action through the , Third pair, 

 although no sensation accompany it. In either view, the rarity of the occur- 

 rence is at once accounted for ; since in most cases of amaurosis, the disease 

 lies in the trunk of the nerve, and thereby checks both its spinal and its cerebral 

 actions. 



206. The physiologist has not at present any knowledge of any similar 

 protective movements, in the Human being, designed to keep the organ of 



