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. 

 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. 



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 



