OF THE EYE MUSCLES IN MARSUPIALS. 327 



The distal end of the trochlear nerve (figs. 3-5, IV.) runs 

 forwards and outwards into the posterior side of the medial end 

 of the m. obliquus supei'ior. 



The naso-ciliary branch of the trigeminal nerve (figs. 3-5, V.) 

 passes out immediately lateral to the oculomotor, downwards 

 below the posterior side of the m. rectus superior, and forwards 

 above the optic stalk and along the anterior lower border of the 

 medial end of the m. obliquus sujaerior to the snout. 



Foetus of G.L. 17 and of 17*5 mm. 



This stage consists of two pouch young, one measuring 17 mm. 

 and the other 17'5 mm. The optic stalk has quite disappeared and 

 the eye muscles have practically attained their adult positions. 



The distal end of the m. obliquus superior turns posteriorly 

 and comes to lie so close to the outer end of the m. rectus superior 

 that the two muscles are inserted almost at the same place. 



The m. rectus internus has increased greatly in size and is 

 now a well-developed muscle ; it is, however, still connected witli 

 the m. rectus inferior at its proximal end by a thin band of cells. 



The m. obliquus infei'ior, as in our last stage, runs obliquely 

 backwards from the ali-nasal cartilage, its posterior end now 

 being inserted anterior to and on the lateral side of the m. rectus 

 externus. 



The m. rectus externus and the m. retractor bulbi are now 

 separate muscles. The former is inserted on the posterior side 

 of the bulbus ; at about its mid-region on the anterior side, an 

 indentation is present which in all probability represents the 

 first indication of the later division of the muscle into two portions. 

 The m. retractor bulbi has developed considerably ; it extends 

 from the middle of the basisphenoid cartila,ge, to which it is 

 becoming attached, anteriorly and outwards, its distal end 

 surrounding the posterior side of the optic nerve in the shape of 

 a half moon, the pointed ends of which stretch out, the one above, 

 the other below the nerve. The m. retractor bulbi is innerv^ated 

 by a small branch from the abducens nerve before the latter runs 

 into the m, i-ectus externus. 



Fcetus of G.L. 5-2 cm. (Text-fig. 19.) 



The m. retractor bulbi (text-fig. 19, r.b.) has now attained its 

 adult position. It enters the orbit on the posterior side of the 

 optic nerve and, surrounding the latter, stretches outwai-ds as a 

 circular sheet of muscle gradually diminishing in thickness and 

 increasing in circumference up to its insertion round the inner 

 side of the bulbus within the recti muscles. The two edges of 

 the sheet lie close together, especially at its outer end, but they 

 never completely fuse. 



The proximal end of the m. rectus externus (text-fig. 19, r.e.) 

 is split up into two portions, already indicated at 17 mm., which 



