MOTOR NERVES OF THE ORBIT. 185 



ducens nerve, from its being solely distributed to the Rectus externus muscle. 

 There is no reason to believe that the actions of either of the two last-mentioned 

 nerves are ever involuntary; on the contrary, there will appear reason to 

 suppose that they are, with a branch of the third pair, the sources of the 

 voluntary movements of the eyes. Cases occasionally present themselves in 

 which this nerve alone is paralyzed; and the outward motion of the ball is 

 then almost entirely lost. 



249. The three other Recti muscles, together with the Levator Palpebra3, 

 and Inferior Oblique, are supplied by the Third pair, commonly termed Oculo- 

 motor. The general question, how far this nerve is to be regarded as exclu- 

 sively motor, has been already considered ( 224); that it is chiefly so, there 

 can be no doubt. But we have now to inquire, whether there is any ground 

 for believing that different branches of the nerve are subservient to motions of 

 a different character some, for example, being more connected with the 

 Reflex function of the Spinal Cord; others with that instinctive tendency 

 which causes opposite muscular actions to take place in the two orbits by one 

 effort of the will; and others being immediately directed and controlled by the 

 will. It will be remembered that this nerve subdivides into two principal 

 branches ; of which one supplies the Levator Palpebra3 and Superior Rectus; 

 whilst the other is distributed to the Internal and Inferior Recti, and to the 

 Inferior Oblique. Now the action of the former appears to be of a purely 

 voluntary character. We have no instance of the upper lid being elevated by 

 any other than an effort of the will ; and, if this be suspended, the Orbicularis 

 may be made to depress it, by the reflexion of a stimulus applied to the edge 

 of the tarsi. Moreover, when a strong light causes the lids to contract invo- 

 luntarily, we feel conscious that a voluntary effort is required to keep them 

 apart. The same may be said of the directly upward movement of the eye- 

 ball, w r hich is caused by the Superior Rectus alone: it is never any thing but 

 a voluntary act; for the upward and inward movement adverted to by Sir C. 

 Bell, is evidently occasioned by the inferior oblique acting alone. On the 

 other hand, it is certain that some, at least, of the actions of the second branch 

 are of a simply reflex nature ; and that others cannot be said to be voluntary, 

 but are rather of an instinctive character. It is from this branch that the 

 twigs proceed, which enter the ciliary ganglion, and which govern the move- 

 ments of the pupil ; movements which have been already shown to be of a 

 simply reflex nature. Some have attempted to show, that the actions of the 

 iris are in a slight degree voluntary, because, by an effort of the will, they 

 could occasion contraction of the pupil ; but this so-called voluntary contraction 

 is always connected with a change in the place of the eyeball itself, occasioned 

 by an action of some of its muscles. It is principally noticed under the two 

 following conditions. 1. When an object is brought very near the eye, and 

 we steadily fix our attention upon it, the axes of the two eyes are made to 

 converge ; and if this convergence be carried to a considerable extent, so that 

 the pupils of both eyes are sensibly directed towards the inner canthus, a 

 contraction of the pupil takes place. The final cause or purpose of this con- 

 traction is very evident. When an object is brought near the eye, the rays 

 proceeding from it would enter the pupil (if it remained of its usual size) at 

 an angle of divergence so much greater than that which would allow them to 

 be properly refracted to a focus, that indistinct vision would necessarily result. 

 By the contraction of the pupil, however, the extreme or most divergent rays 

 are cut off, and the pencil is reduced within the proper angle. The principle 

 is precisely the same as that on which the optician applies a stop behind his 

 lenses, which reduces their aperture in proportion to the shortness of their 

 focal distance. 2. Contraction of the pupil is also noticed, when the eyeball 

 is performing that rotation upwards and inwards which has been already 



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