CHAP, in.] SIGHT. 1283 



wards is inserted into the sclerotic underneath the external rectus 

 in the hind temporal part of the ball. The line of insertion 

 (Fig. 155) is also an oblique one like that of the superior oblique 

 but it is placed somewhat farther past it ; its hind end lies not 

 far from the entrance of the optic nerve and it runs thence 

 forwards and downwards. 



791. The manner in which these muscles are thus severally 

 attached to the eyeball suggests that in contracting they would 

 move the eyeball in the following ways. Taking changes in the 

 direction of the visual axis as indicating the nature of each 

 movement we should expect that the superior rectus would turn 

 the visual axis upwards, the inferior rectus downwards, the ex- 

 ternal rectus outwards towards the temporal side, and the internal 

 rectus inwards towards the nasal side. The inferior oblique, its 

 insertion being on the hind and lateral part of the eyeball, and 

 the direction of the muscle being downwards, would in contracting* 

 turn the visual axis upwards, while the superior oblique having 

 a somewhat similar insertion but acting in an opposite direction 

 would turn the visual axis downwards. Both muscles however 

 in thus raising or lowering the visual axis would, owing to the 

 oblique direction of their insertions at the same time, turn it 

 to the temporal side ; the movement, as the names of the muscles 

 suggest, would be an oblique one. 



The six muscles therefore would seem to act as three pairs, 

 the superior and inferior rectus, the internal and external rectus, 

 and the inferior and superior oblique, each pair rotating the eye- 

 ball round a particular axis. Calculations based on a careful 

 study of the attachments and directions of the several muscles, 

 and the results of actual observations, shew that this is so, and 

 that the movements carried out by the several pairs may be more 

 accurately described as follows. 



The superior rectus and the inferior rectus (see Fig. 156} 

 rotate the eye round a horizontal axis, which may be described 

 as one directed from the root of the nose to the temple; it is 

 therefore not a line at right angles with the visual axis but one 

 making an acute angle (20) with such a line. The superior and 

 inferior oblique rotate the eye round a horizontal axis which may 

 be described as one directed from the centre of the eyeball to- 

 the occiput; it again is not a line at right angles to the visual axis, 

 but makes an angle, with such a line, larger (60) than the similar 

 angle made by the inferior and superior rectus, and turned in a 

 different direction. The internal rectus and external rectus rotate 

 the eyeball round a vertical axis passing through the centre of 

 rotation of the eyeball parallel to the median plane of the head 

 when the head is vertical; this therefore is at right angles to- 

 the visual axis, and so differs from the other two. 



When we compare the movements thus effected by these 

 several pairs of muscles with the movements which we described 



F. 82 



