326 MISS E. A. FRASER ON THE DEVELOPMENT 



gradually as it proceeds below the bulbus, its pointed ventral 

 end being inserted close to the m. obliquus inferior. Near its 

 wide proximal end, immediately in front of the optic stalk, the 

 m. rectus internus branches off and runs outwards with a slightly 

 dorsal trend on to the anterior side of the bulbus close below the 

 m. obliquus superior (figs. 3 & 5, r.i.). The m. rectus internus 

 is as yet a much thinner and shorter muscle than the m. rectus 

 inferior but has nevertheless grown considerably since our last 

 stage, now stretching farther laterally and upwards towards its 

 future point of insertion in the eyeball. 



The m. obliquus inferior (PL II. figs. 3-5, o.inf.) is no longer 

 connected with the m. rectus infei'ior. It appears as a more or 

 less dorso- ventral ly flattened structure of a somewhat oblong 

 shape, having a slightly longer axis in an obliquely antei'O- 

 posterior direction ; as at 13"5 mm., its now proximal but formally 

 distal end is attached to the ali-nasal cartilage, the opposite 

 once proximal end extending towards the lower anterior side of 

 the bulbus. 



As regards the abducens complex, we see at 15 mm. veiy little 

 advance compared with the last stage. The portion corresponding 

 to the m. retractor bulbi (figs. 4 & 5, r.h.) stretches farther 

 inwards and posteriorly than any of the other muscles ; it 

 increases in thickness as it runs antero-laterally and downwards 

 towards the outer end of the optic stalk, its distal end lying just 

 behind the junction of the oculomotor with the ciliary ganglion. 

 In the model this end shows the first indications of growth round 

 the posterior side of the optic stalk, but this growth round the 

 stalk is, however, more marked in 14 mm. and 14*5 mm. The 

 m. rectus externus (figs. 4 & 5, r.e.) stretches from the lateral 

 side of the m. retractor bulbi, not far from the anterior end of 

 the latter, outwards and downwards, its outer pointed end lying 

 below the postero-venti'al side of the bulbus some distance 

 behind the outer end of the m. obliquus inferior. 



The abducens nerve extends forwards from its origin and then 

 downwards on the outer side of the m. retractor bulbi, running 

 into the abducens complex at the region where the two components 

 of the latter are joined with one another (fig. 4, VI.). 



The oculomotor nerve (PI. II. figs. 3-5, III.) runs forwards 

 medial to the naso-ciliary branch of the trigeminal into the 

 posterior end of the m. rectus superior, and continvies obliquely 

 outwards and downwards close against the inner end of the 

 ciliary ganglion and along the side of the m. rectus inferior next 

 the eye. It gives off fibres on this same side into the root of the 

 m. rectus internus. Leaving the m. rectias inferior before the 

 latter reaches the bulbus, it passes anteriorly and ventrally into 

 the m. obliquus inferior. 



The ciliary ganglion forms an elongated mass of cells stretching 

 out below the optic stalk ; at 15 mm., as seen in the model (fig. 5, 

 cil.g.), it is not so elongated as in some of the examples measuring 

 13 mm. and 14 mm. 



