SENSE OF SIGHT. , 453 



movement. Another group of two muscles exists, whose 

 office it is to produce in the globe a movement of rotation 

 upon its antero-posterior axis. These are the two oblique 

 muscles. By careful examination of the points of insertion 

 or reflection of these muscles (the pulley (trochlea) of the 

 trochlearis, or superior oblique muscle) we find that they 

 both serve to direct the pupil outwards, and also to produce 

 in it a rotary movement, the direction of which, in the right 

 eye, for instance, under the influence of the superior oblique 

 muscle, will be the same as that of the hands of a watch, 

 and the reverse, when under the influence of the small 

 oblique muscle. The purpose of these rotary movements 

 appears to be to counterbalance those of the head, and to 

 maintain the parallelism of the two eyes, when the head is 

 bent on one side or the other. 



The oblique muscles also extend from the front to the back, 

 being inserted in the posterior hemisphere of the globe of the 

 eye ; they thus draw the globe forwards, and when this 

 movement coincides with that of the recti muscles, which 

 draw the globe slightly backwards, and, especially, with that 

 of the orbicularis palpebrarum, which compresses it from 

 front to back, a sort of compression of the globe of the eye is 

 the result: this compression is intended to prevent too 

 violent congestion of the eye, which is thus compressed as a 

 person would compress a sponge. In the same way, when 

 making violent efforts which send the blood to the head, we 

 instinctively close our eyes, and forcibly contract the muscles 

 attached to them; children who scream so violently that 

 their face becomes suffused with blood, shut their eyes tight 

 while doing so, and, no doubt, at the same time contract the 

 oblique muscles. 1 



1 See, on this subject, some extremely original ideas of Dar- 

 win's on the movements of the face in regard to the expression of 

 painful and sad emotions. " When children scream loudly, the 

 action of screaming produces a great change in the circulation, the 

 blood being carried to the head, and especially to the eyes, pro- 

 ducing a disagreeable sensation. Charles Hell has observed that, 

 under these circumstances, the muscles which surround the eyes 

 contract in such a manner as to protect them. This action has 

 become, by the effect of natural selection and inheritance, an in- 

 stinctive habit. As man advances to mature age, he learns to con- 

 trol, in a great measure, the disposition to cry out, having found 

 indulgence in it painful; he is thus able to avoid the contraction of 

 the corrugator muscles, but can prevent that of the pyramidal 



