306 FUNCTIONS OF THE NERVOUS SYSTEM. 



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 sen- 

 sation directs it. Further, it explains the almost irresistible nature of the tend- 

 ency to winking, which is performed at short intervals 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. 



395. 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 neces- 

 sary; for, even when the sight of both eyes has been entirely destroyed by 

 amaurosis, 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 stimulus 

 is not that of light conveyed through the Optic nerve; but that of heat con- 

 veyed 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 Sensorium, ordinarily produces a sensation; 

 whilst in these curious cases, no sensation is produced, on account of a dis- 

 ordered state of the part of the ganglionic centre in which the Optic nerve 

 terminates; though some filaments of that nerve, being connected with the 

 Third pair by means of a distinct tract of vesicular matter, can produce a 

 reflex action through it, although no sensation intervene. In either view, the 

 rarity of the occurrence 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 encephalic actions. 



396. The Physiologist has not at present any knowledge of any similar 

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

 ing from injury ; but there can be little doubt that those which we are constantly 

 witnessing in other animals, possessing large external ears, are reflex actions 

 excited by the irritation applied to them. In regard to the Nose, we find a 

 remarkably complex action that of Sneezing adapted to drive off any cause 



