216 The Vertebrate Organ Systems 



with small hairs, the eyelashes, which can catch small particles of dust. 

 The inner portions of the eyelids are lined with the conjunctiva, a 

 mucous membrane, which is also reflected over the eye. Inside the 

 eyelid it is reddish in color, but over the eye itself, the conjunctiva is 

 transparent. The conjunctiva makes possible the sealing ofif of the 

 entire eyeball from the possible penetration of foreign bodies in or 

 around it. At the inner angle of the eye, the conjunctiva forms a verti- 

 cal fold. In some vertebrates (the frog, for example) this is extended 

 as a third eyelid or nictitating membrane. Between the conjunctiva 

 and the skin of the eyelid is a heavy fibrous layer containing numerous 

 oil-secreting glands, the meibomian glands. Their oily secretion is 

 emptied at the base of the eyelashes and serves to seal the eyelids to the 

 surface of the eyeball as they move up and down. 



The eyes are constantly washed and kept clean by the secretions 

 of the tear or lacrimal glands. These glands are located in a depression 

 of the frontal bones along the outer margin of the orbit. They empty 

 by means of several small ducts under the upper eyelids. The tears 

 flow across the front of the eye and drain into the small lacrimal lake 

 at the inner margin. The tears are then discharged by means of the 

 nasolacrimal duct into the nasal passageways. 



The movements of the eye are made possible by a series of six 

 muscles which are able to move the eyeball from side to side and up 

 and down. These muscles have their origin on the bony socket and 

 their insertion on the heavy outer coat of the eyeball. 



Most of the wall of the eyeball is composed of three distinct coats : 

 the outer fibrous or sclerotic; the middle vascular or choroid; and the 

 inner nervous or retina. 



The sclerotic coat is formed of a heavy, opaque, fibrous, connective 

 tissue and surrounds the entire eyeball. At the anterior portion, part 

 of this coat becomes transparent to form the cornea. The white of the 

 eye is a continuation of the opaque portion of this coat. 



The intermediate layer, the choroid coat, is a thin layer formed of 

 numerous anastomosing capillaries and pigment cells. The capillaries 

 are important for the maintenance of the metabolism of the eye, while 

 the pigment cells provide a dark background for the light-sensitive cells 

 of the retina. Their function is similar to that served by the dark 

 coating or backing on a film : they prevent light from reflecting back 

 and giving distorted images. Like the sclerotic coat, this coat is also 

 highly modified at the front of the eyeball. Here it is modified into 

 the pigmented iris and the ciliary body. In the center of the iris 



