346 FUNCTIONS OF THE NERVOUS SYSTEM. 



duction, of the cornea, as a result of the action of the Oblique muscles ; all 

 these movements being attributable to the Recti alone. 



452. 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 har- 

 monious; 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 ac- 

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

 namely, the direction of both eyes towards the requirefl objects. In order to 

 study them properly, it is necessary to reduce them to some kind of classifica- 

 tion. We may divide them into the Voluntary and the Involuntary; and the 

 former, being numerous, require to be further classified. They may be ar- 

 ranged under two groups ; the first comprising those which are alike har- 

 monious and symmetrical ; the second including those which are harmonious 

 but not symmetrical. To the first group belong the following: 1. Both 

 eyeballs are elevated by the contraction of the two Superior Recti. 2. Both 

 eyeballs are depressed by the conjoint action of the Inferior Recti muscles. 

 3. Both are drawn directly inwards, or inwards and downwards, as when 

 we look at an object placed on or near the nose ; this movement is effected by 

 the action of the Internal Recti of the two sides, with or without the Inferior 

 Recti. It is evidently symmetrical, but might seem at first sight not to be 

 harmonious, because the eyes do not move together towards one side or the 

 other; it is, however, really harmonious, since their axes are directed towards 

 the same point. Now it is to be observed, with regard to these movements, 

 that we can never effect them in antagonism with each other, or with those of 

 other muscles. We cannot, for example, raise one eye and depress the other; 

 nor can we raise or depress one eye, when we adduct or abduct the other. 

 The explanation of this will be found in the fact, that we can never, by so 

 doing, direct the eyes to the same point. The harmonious but unsymmetrical 

 movements, forming the second class, are those in which the Internal and Ex- 

 ternal Recti of the two sides are made to act together, either alone, or in con- 

 junction with the Superior and Inferior Recti. They are as follows. 4. One 

 eye is made to revolve directly imvards, by the action of its Internal Rectus, 

 whilst the other is turned outwards by the action of its External Rectus. 5. 

 One eye is made to revolve upwards and inwards, by the conjoint action of 

 the Internal and Superior Recti ; the other, itpwards and outicards, by the 

 conjoint action of the External and Superior Recti. 6. One eye is made to 

 revolve downwards and inwards, by the conjoint action of the Internal and 

 Inferior Recti ; the other, downwards and outivards, by the conjoint action 

 of the External and Inferior Recti. In these movements, two different mus- 

 cles, the Abducens and Adducens, are called into action on the two sides ; but 

 they are so employed for the purpose of directing the axes of the eyes towards 

 the same point. 



453. The normal Involuntary movements of the eyeballs are only of two 

 kinds. 1. The rotation of the two eyeballs on their own axes, which takes 

 place when the head is moved in certain directions ( 451) ; this is effected 

 in direct respondence to certain guiding sensations, and without any influ- 

 ence or control on the part of the will ; it is therefore a purely consensual 

 action. 2. The revolution of both eyes n/i/rart/n and inward*, which takes 

 place in the acts of coughing, sneezing, winking, &c. ; this is altogether inde- 

 pendent of visual sensations, and is commonly, like the other movements 

 associated in these actions, of a reflex nature. Many abnormal movements 

 of the eyeballs, in which there is neither harmony nor symmetry in the 

 actions of the muscles, present themselves in convulsive diseases. 



454. It may be stated as a physiological fact, that Single Vision with two 

 eyes is dependent upon the formation of the image upon parts of the two 



