68 THE ORGANS OF SPECIAL SENSE. 



edges turn the eye inward and outward; and the superior 

 and inferior oblique rotate the eye. The nerves supply- 

 ing these muscles are the third or motor oculi, the fourth 

 and the sixth. 



The lachrymal gland, which is about the size and 

 shape of an almond, is situated at the upper and outer 

 part of the orbit. It secretes a fluid which keeps the 

 anterior surface of the eye bathed in moisture and is 

 ordinarily drained away through the lachrymal sac in 

 the inner canthus, whence it passes by the lachrymal 

 ducts into the nose. When the amount secreted is 

 excessive, it overflows the lower lid as tears. 



Lacrimal gland. 

 Tarsal cartilage. 



Nasal or tear-duct. 



Canaliculus. 

 FIG. 24. Diagram of the lacrimal apparatus. (Pyle.) 



Coats of Eye. The membranes or coats of the eye 

 are three in number: an outer or sclerotic, a middle or 

 vascular, and an inner or sensitive. 



The sclerotic coat is a rather thick, fibrous, protect- 

 ive membrane. Where it passes in front of the iris, 

 however, it is thinner and transparent and is known 

 as the cornea. The cornea projects somewhat and, as 

 it were, resembles a segment of a smaller sphere set into 

 the rest of the sclerotic. 



The middle or vascular coat, known as the choroid, 

 carries blood-vessels for the retina or sensitive coat in 

 its inner layer and has an outer layer of pigment cells 

 that excludes light and darkens the inner chamber of 

 the eye. The folds of the choroid at its anterior mar- 



