186 FUNCTIONS OF THE NERVOUS SYSTEM. 



spoken of as occasioned by the contraction of the Inferior Oblique muscle ; 

 and which, when performed along with violent respiratory actions, or during 

 sleep, must be regarded as involuntary. This rotation also takes place, to a 

 slight degree, when the eyelid is depressed, as in ordinary winking; and it 

 is obvious that, in this manner, the surface of the eye is more effectually swept 

 free from impurities which m&y have gathered upon it, than it would be by 

 the downward motion ofthe lid alone. But the pupil is not contracted when 

 the eyeball is voluntarily rotated upwards and inwards, an action which 

 \rnay be effected by the Superior Rectus, some fibres of which are sufficiently 

 rar removed from the central axis of the globe to give it an internal direction. 

 There is good reason to believe, therefore, that the actions of the inferior 

 branch of the Third nerve are in great part automatic, whilst those of the 

 superior branch are purely voluntary. Upon this reasoning, Valentin has 

 founded a very ingenious theory of the consensual movements of the eyeball, 

 which will now be explained ; and this w r ill be conveniently followed by an 

 inquiry into the nature of this class of movements, as distinguished from the 

 Reflex on the one hand, and the Volitional on the other. 

 / 



XVI. Consensual Movements of the Eye. 



^ * 



250. From the foregoing observations it appears, that the Rectus Superior, 

 Obliquus Superior, and Rectus Externus, which are supplied from the superior 

 branch of the Third pair, and by the Fourth and Sixth pairs, are all to be 

 regarded as purely voluntary muscles ; and Valentin considers them analogous 

 to the Extensors of the limbs, spine, &c., which are for the most part distin- 

 guished by the same character. By the actions of these three muscles, singly 

 or combined, the eyeball may be moved in nearly all directions. On the 

 other hand, the Inferior and Internal Recti, and the Inferior Oblique, supplied 

 by the inferior branch of the Third pair, are more or less automatic in their 

 action ; and these are compared, by Valentin, to the flexors. By the single 

 or combined actions of these muscles also, the eyeball may be moved towards 

 almost any point, except in an upward and outward direction ; and any one 

 who tries the experiment will find that this is, of all the movements of the 

 eye, the one that is attended with the most constrained action of the muscles. 



251. On studying the conjoint movements of the eyeball, we are led to 

 observe^ the very curious fact, that they are not so much symmetrical as 

 harmonious ; that is to say, the corresponding muscles on the two sides are 

 rarely in action at once ; whilst such a harmony or consent exists between 

 the actions of the muscles of the two orbits, that they work to one common 

 purpose, namely, the direction of both eyes toward the required object. In 

 order to study them properly, it is necessary to reduce them to seme kind of 

 classification. 1. If one eye be rotated imvards, and the other outwards, the 

 Internal Rectus of one eye, and the External Rectus of the other, a^e evidently 

 put in action together. This movement is harmonious or consensual, but not 

 symmetrical. 2. Both eyeballs are elevated, by the contraction of the two 

 Superior Recti. 3. Both eyeballs are depressed; this is effected by the con- 

 joint action of the Inferior Recti muscles ; and the movement is, like the preced- 

 ing, both harmonic and symmetrical. 4. Both are drawn directly inwards 

 and downwards, as when $e look at an object placed on or near the nose ; this 

 movement is symmetrical,* }>ut not harmonic ; and it is effected by the action 

 of the Internal Rectus, joined either with the Inferior Rectus or the Superior 

 Oblique. 5. When one eye is rolled upwards and inwards, and the other 

 upwards and outwards, the Inferior Oblique is probably operating on one side, 

 whilst the Superior Rectus unites its action with that of the External on the 

 other. And, 6, when one eye is drawn downwards and inwards, and the 



