422 SPECIAL PHYSIOLOGY. 



tively, by the inferior and superior oblique muscles ; the movements 

 around the antero-posterior axis, are exceedingly slight, and are per- 

 formed by the aid of the oblique muscles. In these simple movements, 

 the eyeball may practically be regarded as a sphere turning round its 

 centre as a nearly fixed point. But besides these movements, the eye- 

 balls are capable of executing oblique motions, as, e. g., upwards and 

 outwards, upwards and inwards, downwards and outwards, and down- 

 wards and inwards ; in such movements, the eyeball moves around 

 intermediate secondary axes, formed by the junctions of two others, 

 and the movements are executed by three muscles viz., by two of the 

 recti with one of the oblique. Finally, with these secondary move- 

 ments, and also with the primary ones, are usually combined the very 

 slight rotatory movements of the eyeballs around their antero-posterior 

 axes, so producing tertiary movements. In this way, the antero-pos- 

 terior axis, and therefore, the centre of the cornea, and the centre of 

 the retina, may describe either straight or curved lines over the field 

 of vision, from one point to another, in every conceivable way as for 

 example, when we trace the contour of a very complicated figure. 

 Elite's ophthalmotrope is an instrument consisting of a movable ball, to 

 which are fixed elastic cords in a state of slight tension, representing 

 the various muscles of the eyeball ; the amount of shortening or elon- 

 gation of the cords, in any given portion of the ball, is taken to indi- 

 cate the actions of the respective muscles. 



The movements of the eyeball are undoubtedly voluntary, but they 

 present certain peculiarities of very great interest. Thus, the move- 

 ments of the two eyeballs are always simultaneous and definite; they 

 are always harmonious, but very frequently not symmetrical. In 

 looking upwards or downwards both eyeballs move harmoniously and 

 symmetrically, the same muscles being called into play in each orbit ; 

 in looking to the right or to the left, the eyeballs move harmoniously, 

 but unsymmetrically, different muscles acting on the two sides. In 

 oblique movements to one or other side, the motions are unsymmetri- 

 cal, being produced, for example, by the superior and external recti 

 of one side, and the superior and internal recti of the other; in 

 rotatory movements, the actions, though harmonious and wonderfully 

 exact, are unsymmetrical, being executed by aid of the superior 

 oblique of the one eye, and the inferior oblique of the other ; lastly, 

 in convergence of the two eyeballs to look at a near object, the action 

 is both harmonious and symmetrical, the internal recti muscles being 

 called into play in each orbit. Again, it is to be remarked, that the 

 movements of the eyeballs are voluntary, and their muscles decidedly 

 under the influence of the will ; yet their motions are limited by a 

 certain kind of combination, which prevents us from acting upon them 

 in a wholly independent way on the two sides r as we can, for example, 

 with our arms and hands. Thus, we cannot turn one eyeball up and 

 the other down ; nor both eyes outwards ; nor can we depart from a 

 certain fixed degree of convergence of the eyes, required for their 

 accommodation to see a given object. The reason of this is, that the 

 movements of the eyes, though voluntary, are guided indirectly by 

 the purposes we strive to attain, which we shall hereafter see are 



