418 



THE NERVES AND THE SENSE ORGANS 



glands (Fig. 301). These secrete tears, and pour them over the eyes. 

 The tear glands are in the upper, outer corners of the eyes. Their 

 secretions are carried by a duct, or tube, into the nostrils. When the 

 tears are secreted in larger amounts than 

 the ducts can carry off, we "cry": the tears 

 overflow the lids. 



Lachrymal 

 Gland 



Nasal 

 Cavity 



Fig. 301. 



Relation of a Lachrymal 

 Gland to the Eye. 



417. Accommodation. The crys- 

 talline lens and cornea together form 

 the most important focusing part 

 of the eye. The crystalline lens 

 differs from a glass lens in that it 

 is very elastic; if left to itself, it 

 becomes more convex. On this fact 

 accommodation depends. The lens 

 is held to the choroid coat by a ligament called the sus- 

 pensory ligament. Ordinarily this ligament is stretched 

 tight by the choroid coat, and flattens the lens. When the 

 lens needs to be more rounded, the ciliary muscles, which 

 are attached to the sclerotic coat, draw the choroid coat 

 forward. The result is that the suspensory ligament is 

 loosened, and the lens is allowed to become more convex. 

 This power of the eye to change the curvature of the lens 

 is accommodation. 



The need for accommodation is plain if we know the properties of 

 light. When light from a distant object, such as a tree, comes to us, 

 the light rays are almost parallel; hence the lens, though flattened, can 

 bring the light to a focus on the retina (cf. Fig. 153, 178). But when 

 you look at a near object, the light rays from any point of the object 

 spread out, and make a large angle with one another; hence a lens 

 with a more rounded surface is needed to bring the rays to a focus. 

 As we become older, the crystalline lens loses its elasticity, and does 



