MOTOR NERVES OF THE ORBIT. 



183 



[Fig. 42. 



bismus, by the operation which has now been so extensively and (when exe- 

 cuted with care and judgment) so successfully performed. 



245. It will be recollected that, in the Human Orbit, six muscles for the 

 movements of the eyeball are found; the four recti, and the two oblique mus- 

 cles. The precise actions of these are not easily established by experiment 

 on the lower animals; for in all those which ordinarily maintain the horizontal 

 position, there is an additional muscle, termed the retractor, which embraces 

 the whole posterior portion of the globe, and passes backwards to be attached 

 to the bottom of the orbit. This muscle is most developed in Ruminating 

 animals, which, during their whole time of feeding, carry their heads in a 

 dependent position. In most Carnivorous animals, instead of the complete 

 hollow muscular cone, (the base enclosing the eyeball, whilst the apex sur- 

 rounds the optic nerve,) which we find in the Ruminants, there are four dis- 

 tinct strips, almost resembling a second set of recti muscles, but deep-seated, 

 and inserted into the posterior instead of the anterior portion of the globe. It 

 is obvious that the actions of these must greatly affect the results of any opera- 

 tions which we may perform upon the other muscles of the Orbit ; and, as it 

 is impossible to divide the former, without completely separating the eye from 

 its attachments, we have no means 

 of correcting such results but by 

 reasoning alone. Experiments upon 

 animals of the order of Q,uadru- 

 mana, most nearly allied to Man, 

 would be more satisfactory, as in 

 them the retractor muscle is almost 

 or entirely absent. If the origin 

 and insertion of the four Recti mus- 

 cles be examined, however, no doubt 

 can remain that each of them, act- 

 ing singly, is capable of causing the 

 globe to revolve in its own direction 

 the superior rectus causing the 

 pupil to turn upwards the internal 

 rectus causing it to roll towards the 

 nose and so on. A very easy and 

 direct application of the laws of me- 

 chanics w r ill further make it evident 

 to us, that the combined action of 

 any two of the Recti muscles will 

 cause the pupil to turn in a direc- 

 tion intermediate between the lines 

 of their single action, and that any 

 intermediate position may thus be 

 given to the eyeball by these mus- 

 cles alone. This fact, which has 

 not received the attention it de- 

 serves, leads us to perceive that the 

 Oblique muscles must have some 

 supplementary function' It may be objected that this is a theoretical statement 

 only, and that there may be some practical obstacle to the performance of 

 diagonal movements by the Recti muscles, w T hich renders the assistance of 

 the Obliques essential for this purpose. But to this it may be replied, that no 

 single muscle can direct the ball either downwards and inwards, or upwards 

 and outwards, and that, as we have good reason to believe these movements to 



A view of the Third, Fourth and Sixth pairs of Nerves; 

 1, ball of the eye and rectus externus muscle ; 2, the 

 superior maxilla; 3, the third pair, or motores oculi, 

 distributed to all the muscles of the eye except the su- 

 perior oblique and external rectus; 4, the fourth pair, or 

 pathetici, going to the superior oblique muscle; 5, one 

 of the branches of the seventh pair; 6, the sixth pair, 

 or motor externus, distributed to the external rectus 

 muscle; 7, spheno-palatine ganglion and branches; S, 

 ciliary nerves from the lenticular ganglion, the short 

 root of which is seen to connect it with the third pair.] 



