372 JAMES ROLLIN SLONAKER 



ber of movements was forty per minute, but when placed so 

 that the eye was m direct sunUght the rate was almost doubled. 

 When the nictitating membrane is stretched across the eye it 

 is translucent, and the pupil can be readily seen through it. 

 There is no doubt that the bird can perceive the presence of 

 objects when this membrane is stretched over the. eye. The 

 importance of accomplishing, with a semitransparent membrane 

 what is performed in other animals by opaque lids, can be 

 readily appreciated when one considers the very rapid locomo- 

 tion of the bird. When flying through woods or bushes— some 

 birds being credited with a speed of as great as ninety miles an 

 hour — the complete obstruction of vision by opaque lids for 

 even a small fraction of a second would often result in a collision 

 and death to the bird. 



THE BLOOD SUPPLY TO THE EYE 



The blood supply to the eye of the sparrow differs in many 

 respects from that in man. Because of these differences and the 

 small size of the eyeball and accessory parts in the sparrow the 

 blood supply has been difficult to determine. It has been well 

 nigh impossible to use the same names for the branches as are 

 used for man because of the lack of conformity of the branches, 

 but so far as possible, I have employed the names used in human 

 anatomy. In order to get a clear idea of the blood supply to 

 the eye it is necessary to describe the general arrangement of 

 the arteries of the head. 



The two common carotids unite into a single trunk a short 

 distance from the heart, which, just before reaching the head 

 divides into right and left common carotids. Figure 12 is a 

 ventral view of the head and the main branches of the arteries. 

 Each of these common carotids gives off a vertebral artery (V), 

 and then divides into external (CE) and internal (CI) carotids. 

 The three main branches of the external carotid (Ppl, MI, and 

 MS) supply the palatine, inferior maxillary, and superior maxi- 

 lary regions, respectively. 



The internal carotid gives off a large branch, the external 

 ophthabnic artery (Op E), which is directed outward to the 



