HEAD SOMITES AND EYE MUSCLES IN CHELYDRA 151 



Oculomotor group. We have seen that the M. obliquus infer- 

 ior was the first of this group to appear. In the view of the 

 model shown in the figure, this muscle is largely covered by others 

 and to facilitate description it will be taken up last. 



The M. rectus superior has now become large and its end of 

 insertion is in close relation with the end of insertion of the M. 

 obliquus superior. Like the last named muscle it is slender at 

 its distal end and increases in diameter towards its end of origin. 

 It extends in a postero-medial direction and crosses the N. oculo- 

 motorius at its entrance into the cihary ganglion, lying in close 

 contact with the anterior surface of the nerve. In crossing it 

 receives a bVanch from the N. oculomotorius, which enters its 

 dorsal side. It then makes a sharp turn dorsad and again mediad, 

 so that at this stage it lies approximately at right angles to all 

 the other muscles of the eye. The M. rectus superior, it will 

 be noticed, is now developmentally in advance of the other eye- 

 muscles, having most nearly reached its definitive position and 

 relations. 



The nasociliary branch of the ophthalmic nerve dips under the 

 M. rectus superior just laterad of the point where it crosses the 

 N. oculomotorius, and takes its course anteriorly, following 

 closely the median surface of the eye-ball. 



The Mm. rectus inferior and rectus medialis form one elongate 

 mass extending antero-ventrally over the medial surface of the 

 eye, crossing the optic stalk ventral to its junction with the 

 eye-ball. Just below its middle an oblique constriction divides 

 the mass into two club-shaped portions with their enlarged ends 

 anterior: the dorsal is the M. rectus inferior, and the ventral 

 is the M. rectus medialis. The ophthalmic artery lies against 

 the posterior narrower end of the M. rectus inferior on its medial 

 side. Both muscles have received their branches from the N. 

 oculomotorius. This nerve passes ventrally from the medial 

 border of the ciliary ganglion, curves gently forward, and crosses 

 the attenuated district between the two muscle-masses to pene- 

 trate the M. rectus mediahs on its medial side. Opposite the 

 thickened distal end of the M. rectus inferior it gives off the 

 branch which enters this muscle on its postero-lateral side. 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 14, NO. 2 



