OCR Text |
Show 1879.] EYE-MUSCLES OF MAMMALS. 127 In the Lemures the choanoid appears again as a distinctly muscular slip with the same relative attachment. The superior oblique keeps its anterior position, the posterior border being nearly twice as far from the optic nerve as the anterior is from the cornea. The inferior oblique has also moved forwards, so that in Nycticebus its posterior border is fa from the optic nerve, while its anterior is fa from the cornea. (In the Cebidae it is often five times nearer to the optic nerve than it is to the cornea.) In Lemur some fibres of the superior oblique are inserted in front of the superior rectus (so' in diagram). The optic nerve is considerably to the inner side of the fundus. In Nycticebus there is a further peculiarity in the superior oblique, in that it pierces the tendon of the superior rectus near its inner border to be inserted behind it. Fig. 11. A diagram, from the outer side, of tbe eye of Nycticebus; shows that the inferior oblique has moved further forwards. The following summary therefore appears to be true :- a. In the higher Quadrumana the muscles closely resemble the human muscles in their attachments, and, as was already known, there is no choanoid muscle. b. In the Cercopithecidee, besides other points of difference, there is always some representative of the choanoid. It is interesting to observe that in the higher families the muscle which may be supposed to be ceasing to be useful becomes degenerated and, at last, almost unrecognizable. c. In the Cebidae no trace of choanoid remains, but the superior oblique has moved forwards. d. In the Hapalidae the superior oblique has moved still further forwards, and changed its relation to the superior rectus, while the optic nerve has also moved outwards. e. In the Lemuridee the choanoid is again distinctly present, but the superior oblique has a different relation to the superior rectus and either pierces it or is attached partly in front of it, while the inferior oblique is also moved forwards. |