90 DISSECTION OF THE EYE. 



portion of this muscle lies to the inside of the internal 

 rectus, between it and the inner wall of the orbit ; its 

 fibres arise round the optic foramen with the recti mus- 

 cles, pass outward along the inner wall of the orbit to 

 its upper and outer angle, where they end in a slender 

 tendon which lies in a groove in a piece of cartilage 

 (the trochlea) found at this point. After passing 

 through the groove the tendon bends backward to the 

 eyeball, and is inserted just beneath the tendon of the 

 superior rectus muscle. The trochlea .serves as a pul- 

 ley to change the direction of the pull of the muscle. 



7. The Retractor Bulbi. Lifting up the four recti 

 muscles, or removing them altogether, there will be 

 found beneath four slips of much paler muscle, having 

 the same general direction as the recti muscles. Taken 

 together they form the retractor bulbi, and in some 

 mammals are united to form a single hollow muscle 

 inserted round the circumference of the eyeball. 



DISSECTION OF THE EYEBALL. 



The dissection of the eyeball can be made upon the 

 dog, or perhaps more conveniently upon one of the 

 ordinary slaughter-house animals, the eyes of which 

 can be obtained very easily from a butcher. Of the 

 animals killed by butchers the pig has the best eye 

 for dissection. While it is smaller than that of the 

 ox or sheep, it is quite large enough to make it easy to 

 dissect, and has the advantage of resembling the hu- 

 man eye more closely in size and general shape, and 

 besides, like the human eye, has no tapetum, so that 

 the anatomy of the retina is more clearly seen. The 

 following directions are written especially for the pig's 

 eye, though they can be used for the eyes of other 



