98 THE STRUCTURE OF THE FOWL 



membrane, the conjunctiva, covers the front 

 of the eyeball and the inner surface of the^ 

 eyelids, a continuous secretion of tears keeping 

 the opposed surfaces moist. The lachrymal: ) 

 gland, by which the tears are secreted, lies 

 above and to the lateral side of the eyeball, i 

 The tears are drained away from the front of*, 

 the eyeball by two small canals which lead 

 into a lachrymal sac. Thence they travel ' 

 into the nasal cavity by way of the naso- 

 lachrymal duct. I 



The movements of the eyeballs are effected' 

 by four straight and two oblique muscles, com- 

 parable to those of mammals, except that the 

 superior oblique does not play through a pulley. 



In shape the eyeball of the fowl (Fig. 50) may 

 be said to consist of the segments of two spheres 

 of different curvature, connected by a conical 

 intermediate portion. The posterior segment 

 corresponds to the greater part of the sclera ; 

 the anterior segment is formed by the cornea ; 

 and the conical connection consists of that ' 

 part of the sclera in which the bony scleral ring 

 is developed. The cornea is of horny consist- 

 ence and transparent, and thus offers no obstacle 

 to the passage of light into the interior of the 

 eyeball. The sclera is dense and white, and 



