ACTION OF THE ACCESSORY ORGANS. 247 



Having traced the mode in which the general physiology of vision is 

 effected, and the part performed by each of the constituents of the eye 

 proper, we shall briefly consider the functions of the rest of the visual 

 apparatus, the anatomical sketch of which has been given under the 

 head of accessory organs; and afterwards inquire into the various in- 

 teresting and important phenomena exhibited by this sense. These 

 organs perform but a secondary part in vision. The orbit shelters the 

 eye, and protects it from external violence. The eyebrows have a 

 similar effect; and, in addition to this, the hair, with which they are 

 furnished, by virtue of its oblique direction towards the temple, and by 

 the sebaceous secretion that covers it, prevents the perspiration from 

 flowing into the eye, and directs it towards the temple or root of the 

 nose. By contracting the eyebrows, they can be thrown forwards and 

 downwards in wrinkles; and can thus protect the eye from too strong 

 a light, especially when coming from above. 



The eyelids cover the eye during sleep, and preserve it from the 

 contact of extraneous bodies. During the waking state, this protection 

 is afforded by the instantaneous occlusion of the eyelids, on the antici- 

 pation of danger to the ball. The incessant nictation likewise spreads 

 the lachrymal secretion over the surface of the conjunctiva, and cleanses 

 it; whilst the movement, at the same time, probably excites the gland 

 to augmented secretion. The chief part of the movement of nictation 

 is performed by the upper eyelid ; the difference in the action of the 

 eyelids being estimated, by some physiologists, as four to one. Under 

 ordinary circumstances, according to M. Adelon, 1 it is the levator 

 palpebrae superioris, which, by its contraction or relaxation, opens or 

 closes the eye ; the orbicularis palpebrarum not acting. If the levator 

 be contracted, the eyelid is raised and folded between the eye and 

 orbit, and the eye is open ; if, on the other hand, the levator be relaxed, 

 or spread passively over the surface of the organ, the eye is closed. In 

 this view, the orbicularis muscle is not contracted, except in extraordi- 

 nary cases, and under the influence of volition; whilst the closure of 

 the eye during sleep is dependent upon simple relaxation of the levator. 

 The views of M. Broussais 2 on this subject are more satisfactory. He 

 considers, that the open state of the eye, in the waking condition, 

 requires no effort; because the two muscles of the eyelids are so ar- 

 ranged, that the action of the levator is much more powerful than that 

 of the orbicularis ; and he adduces, in proof of this, that the eyelids, at 

 the time of death, are half open. On the other hand, the closure of 

 the eye in sleep he conceives to be owing to the contraction of the 

 orbicularis muscle, which acts whilst the others rest. If the opening 

 of the eye were wholly dependent upon the action of the levator pal- 

 pebrae superioris, its relaxation during insensibility and death ought 

 to be sufficient to close the eye completely; and the orbicularis palpe- 

 brarum would be comparatively devoid of function ; being only necessary 

 for the closure of the organ under the influence of volition. 



It has been found by experiments instituted by Sir Charles Bell, 3 



1 Physiologie de 1'Homme, 2de edit, i. 419, Paris, 1829. 2 Op. citat., p. 188. 



3 The Nervous System of the Human Body, Amer. edit., p. 48, Washington, 1833. 



