150 HUMAN PHYSIOLOGY 



It is called the cornea. The rest of the eyeball is white 

 and nearly opaque. The eye is lodged in a deep recess 

 of the skull. It has a cushion of fat behind it and 

 six small muscles are inserted in it. The optic nerve 

 runs back to the brain from a point which is not at the 

 center of the eyeball but distinctly to the nasal side. 



The eye is protected by the lids. When these are 

 brought together there is a sac formed which extends 

 over about half the eyeball. It is a mere slit when viewed 

 with reference to its other dimension but it contains a 

 small quantity of tears. Glands under the overhanging 



FIG. 34. The right eyeball with some of its muscles in the shelter 

 of the orbit. 



eyebrow secrete the tears into the sac behind the upper 

 lid. The liquid spreads downward and inward over the 

 surface of the eye to reach two little openings near the 

 inner angle of the lids whence a duct provides for drain- 

 age into the nose. The tears serve to wash the eye and 

 preserve it from drying effects. The response of the 

 glands when a foreign body has struck the cornea is 

 reasonable, but that in emotional crises is hard to ac- 

 count for. 



The eyeball is described as composed of three coats. 

 The outer one is tough and dense; the cornea is a part 

 of it. The middle coat is distinguished by the rich- 



