368 



JAMES ROLLIN SLONAKER 



opening of the duct to the roof of the mouth. In the normal 

 position of the head the secretion would naturally drop this 

 distance directly into the buccal cavity. 



THE EXTRINSIC MUSCLES OF THE EYE 



The muscles which move the eye of the bird in some respects 

 closely resemble those of mammals. As in mammals, they 

 include four rectus and two oblique muscles. Besides these, 

 which function in moving the eyeball, birds possess also a pyra- 



Fig. 9 Enlarged view showing the lacrimal canals exposed by the removal of 

 skin covering them. Ic, lacrimal canal; en, external nares. 



midalis and a quadratus muscle whose contractions draw the 

 nictitating membrane over the front of the eye. 



In the sparrow the ribbon-like rectus muscles have their 

 origin in the thick sheath surrounding the optic nerve, close to 

 the op^tic chiasma. A marked difference is noticed in the rela- 

 tive length of these muscles in birds and manmials, due to the 

 very short optic nerve and the relatively shallow eye socket in 

 the bird. The main part of the muscular tissue of these muscles 

 in birds is situated in the proximal two-thirds of their length. 

 Each muscle widens distally, becomes thin, and finally loses its 

 muscular tissue for the distal third of its course. Distally they 



