THE EYE-MUSCLES. 227 



muscular fibres, a peculiarity which they exhibit from 

 their origin to their insertion. They have, as we shall 

 see, a slightly different innervation. 



The m. rectus internus originates, as before indicated, 

 near the caudal end of the sub-cranial canal, in a groove 

 in the dorsal side of the parasphenoid bone. The muscles 

 of the two sides occupy the ventro-median angle of the 

 canal for its entire length (Fig. 2, r. i. t.) and still farther 

 cephalad in the same relative position on either side of the 

 parasphenoid along the inner sides of the orbits to their 

 insertions on the cephalic borders of the eye-balls. The 

 fibres which compose the dorsal edge of this muscle are of 

 much smaller diameter than the other fibres. 



The m. rectus superior originates in the cephalic end 

 of the sub-cranial canal from the parasphenoid under the 

 m. rectus internus and from the membranous roof of the 

 canal over the same muscle. These two muscles run in 

 close contact (Fig. 2) to the end of the canal, when the m. 

 rectus superior turns dorsad and laterad running over the 

 m. rectus inferior and the optic nerve to its insertion on 

 the eye-ball. The fibres of the dorsal part of the muscle 

 are smaller than the others. 



The m. rectus inferior arises from the basisphenoid 

 and runs over the m, rectus internus, under the m. rectus 

 superior and the optic nerve to its insertion on the eye- 

 ball. The ventral edge of this muscle is composed of 

 smaller fibres than the others. 



The mm. obliquus superior and obliquus inferior 

 arise together far cephalad from the internasal cartilage 

 running caudad to their insertions. In the centre of this 

 massive cartilage a median unpaired horizontal canal (the 

 anterior eye-muscle canal) is excavated, which runs from 

 it caudal face cephalad for a considerable distance into its 



