191 



In the course of this examination he shows that the six muscles, 

 which are attached to the eye-ball, do not, as has been supposed, 

 form one class of voluntary muscles ; but that while the four straight 

 muscles, or recti, are provided for the voluntary motions of the eye 

 when directed to objects, the other two, called oblique, perform 

 certain involuntary motions. These involuntary actions are shown 

 to be a provision for the better protection of the eye ; for when the 

 eye-lids wink and close to wash the cornea, the effect would be in- 

 complete, and the object but imperfectly attained, unless the cornea 

 were at the same time raised by the revolving of the eye-ball. 



After having proved that the eye-ball revolves so as to carry the 

 cornea upwards during the motion of the eye-lids ; and having 

 shown also that the oblique muscles are the agents in this involun- 

 tary and instinctive motion, he proceeds to demonstrate that the 

 same muscles elevate the cornea during sleep. 



The author says, that while we are awake, the eye is under the 

 active influence of the four straight muscles ; but when the eye-lids 

 are closed in sleep these muscles resign their office, and the involun- 

 tary oblique muscles prevail, so as to draw the cornea under the 

 upper eye-lid. This is also shown to be the condition of the eye in 

 faintness and on the approach of death, and on all other occasions 

 when languor or debility prevail over the voluntary muscles of the 

 frame. 



The author notices, incidentally, that the enjoyment of the sense 

 of vision is attended with the excited condition of the recti or vo- 

 luntary muscles, and that insensibility to the impression on the eye 

 is followed by relaxation and neglect of the same class of muscles, 

 and consequently that a depraved and injured condition of the retina 

 is one cause of squinting ; for the oblique prevailing, while the recti 

 or involuntary muscles are neglected, draw the eye so affected from 

 the parallel line of vision. 



After having shown that the recti, or voluntary muscles, are 

 strictly associated with the activity of the retina or organ of vision, 

 he proceeds to express his opinion that the ideas received through the 

 eye are not limited to the office of the retina, but that the sense of 

 vision properly so called, is aided by the sense of voluntary exertion 

 in the recti muscles, and afford us the knowledge of the position and 

 relation of bodies, in addition to the ideas of form, shades, and co- 

 lours, which are received through the retina. 



The paper is illustrated by references to comparative anatomy, and 

 by observations and experiments on man and brutes. 



After having described the variety of actions performed by the 

 muscles of the eye, the author proposes in the second part of the 

 paper to arrange the nerves which go into the orbit according to their 

 offices. 



