THE NERVOUS SYSTEM 377 



5. The Meibomian glands {Plate XXXIX., A 10 10), which are 



found on the margin of the eyelids, and secrete an oily 

 material that prevents them gumming together. 



6. The eyelashes, which are the fine hairs that are found on 



the free edges of the eyelids, and are more abundant 

 on the upper lid than the lower, their purpose being 

 to assist in preventing the entrance of foreign bodies. 



622. The Lachrymal Gland (Plate XXXIX., A 6) is situated 

 on the upper portion of the eyeball, and secretes the tears, which 

 keep the front of the eye moist and clear. The tears pass through 

 the lachrymal duct into the nose. 



623. The Membrana Nictitans, or Haw (Plate XXXIX., 

 A 3), is a cartilaginous structure situated on the inner side of the 

 eyeball. Its function is to remove foreign bodies and shield the eye. 



624. The Eyeball is composed of three coats, viz. : (1) The 

 sclerotic and cornea ; (2) the choroid and iris ; and (3) the 

 retina. 



625. The Sclerotic Coat (Plate XXXIX., B 3 3) is a very 

 dense structure formed of white fibrous tissue, and to it are 

 attached the muscles which move the eyeball. This coat forms 

 four-fifths of the external tunic. 



626. The Cornea (Plate XXXIX., B 2) is that transparent 

 elliptical portion of the external coat situated in the front of the eye. 

 It fits into the sclerotic somewhat similar to the glass in a watch, so 

 forming the remaining fifth of the outer envelope. 



627. The Choroid Coat (Plate XXXIX., B 4 4) consists of 

 three layers — i.e., the external, composed principally of minute 

 veins ; middle, a plexus of small arteries ; and the internal, a dark 

 brown pigmentary layer. 



628. The Iris (Plate XXXIX., A 22 and B 10 10), or curtain, 

 is connected with the choroid, and may be of various colourings. In 



