LESSON 96.] THE EYES IN MAMMALIA AND MAN. 309 



vide, intermingle) with the tubes of the other optic nerve. By this 

 arrangement, an object seen by one eye has its history given, simul- 

 taneously, to that part of the brain with which the other optic nerve 

 communicates, directly through its agency. 



If one eye be closed, by the hand or a bandage, objects seen by 

 the open eye are not sharp and distinct, because it has lost half its 

 ordinary power. 



In looking through Telescopes and Microscopes, it is most impor- 



FIG. 438. 



Longitudinal section of the human eye. 

 a. The sclerotic coat. 

 6. The cornea, connected to the former by means of a bevelled edge. 



c. The choroid, connected anteriorly with 



d. The ciliary ligament, and 



e. The ciliary processes. 

 / The iris. 



g. The pupil. 



h. The third layer of the eye, the retina. 



i. The canal of Petit, which encircles the lens (m); the thin layer in front of this canal is the 



zonula ciliaris. 



k. The anterior chamber of the eye, containing the aqueous humor. 

 I. The posterior chamber. 



m. The lens, more convex behind than before, and enclosed in its capsule. 

 n. The vitreous humor, enclosed in the hyaloid membrane, 

 o. Tubular sheath of the hyaloid membrane, which serves for the passage of the artery of the 



capsule of the lens. 



p. Neurilemma (sheath) of the optic nerve. 

 q. The arteria centralis retina, embedded in its centre. 



tant to keep both eyes open, and this can easily be done by turning 

 the head aside, and thus diverting the axis of vision. Those persons 

 who shut one eye never see an object distinctly, and what is worse, 

 they have created pain in both eyes ; the open one has been strained 

 to do impossibilities, and the closed eye intensely excited but not 

 permitted to be active. If properly managed, the spectator will 

 have a clear view of the object in the microscope, and distinctly see, 

 at the same time, objects on the table, and at a wide angle. 



1406. The ciliary processes may be seen in two ways, either 



