110 CONSCIOUS NERVOUS OPERATIONS 



aqueous humor. (4) The larger cavity of the eyeball, 

 behind the iris and the crystalline lens, called the posterior 

 chamber, is filled with a transparent, semifluid, jellylike 

 substance called the vitreous humor. 



147. The Muscles of the Eyeball and their Nervous Supply 

 (Fig. 66). Each eye is moved by six muscles, four of 



which are straight and two ob- 

 lique. The straight, or rectus, 

 muscles have one end attached 

 to the margin of the opening in 

 the orbit through which the 

 optic nerve and accompanying 

 blood vessels pass, while the 

 Fig. 66. -Muscles of the ot h er j s inserted into the eye- 

 ball. The internal rectus muscle, 



inserted on the nasal side of the eyeball, turns the ball 

 inward ; the external rectus, inserted on the outer or tem- 

 poral side, turns it outward. The superior rectus, inserted 

 on the upper and forward side, pulls the eye upward ; the 

 inferior rectus, inserted on the under and forward part of 

 the ball, draws it down. 



The remaining muscles are called the superior and the 

 inferior oblique, and they unite with the straight muscles 

 to move the eyeballs inward and upward, inward and 

 downward, outward and upward, outward and downward, 

 and to produce a measure of rotation upon an axis. The 

 superior oblique muscle arises near the straight muscles in 

 the edge of .the orbit. Near its forward end it narrows 

 into a tendon which passes through a ring of fibrocartilage 

 attached to a notch in the frontal bone which bounds the 

 front and upper margin of the orbit ; the end of the tendon 

 is then inserted into the upper side of the eyeball. The 

 cartilage ring acts as a pulley to change the direction of 



