432 JAMES ROLLIN SLONAKER 



vision in which each of the foveae is involved is therefore impos- 

 sible. It has already been stated that, in horizontal sections 

 through the whole head, the angle formed by the median plane 

 and the axis of vision w^as about 65°. This evidently represents 

 the angle formed ^vhen the extrinsic muscles are in a resting con- 

 dition. Since the foregoing experiments show that this angle 

 in the living bird may be as small as 25°, we must conclude that 

 the sparrow is able to rotate each eye as much as 40°. This 

 would make the axes of vision form an angle with each other of 

 only 50°. Whether this represents the maximimi amount of con- 

 vergence the sparrow^ is capable of I cannot say. But it does 

 show that this bird can converge its eyes as much as 80° from 

 the resting condition and that a portion of the retina much 

 nearer the fovea may thus function in binocular vision than 

 would be indicated bv the prepared sections. 



A further fact brought out by these experiments is that the 

 sparrow can apparently see directly in front of it. This, coupled 

 with the ability to rotate its eye in the socket, though not suffi- 

 cient to bring the axes parallel, indicates that the third supposi- 

 tion is correct, that is, the sparrow is able to use the temporal 

 part of its retina in binocular vision. That the temporal portion 

 of the retina is more active and expends a greater amount of 

 energy than the nasal part is indicated by the richer blood 

 supply in the chorioid back of it. 



In many other species of birds that part of the retina which 

 would serve in binocular vision is specialized for this purpose. 

 Former investigators (Chievitz '91, '99; Stonaker '97) have shown 

 that these birds possess not only a fovea which functions in monoc- 

 ular vision like the fovea of the sparrow, but a second fovea in 

 each eye so situated in the temporal region of the retina as to 

 receive rays of light from a common object. This second fovea 

 varies from a well-defined depression to one so shallow as to be 

 scarcely noticeable. The powder of acute binocular vision appears 

 to grow less as this temporal fovea becomes more shallow. Al- 

 though the sparrow does not possess a second fovea, the region 

 of the retina where such a fovea would be located is apparently 

 able to serve for this purpose, even though no modification of 



