No. 3-] ACUTE VISION IN VERTEBRATES. 



453 



Fig. 5. — White-bellied Swallow (Tachycineta bicolor) 3/1. 



NH and NI. Axes of vision of fovea nasalis. 

 TR and TL. " " " temporalis. 



along the axis of vision. I have not attempted an exact repre- 

 sentation of the area, but have only indicated by dotted lines 

 its position and extent as I have found it (PI. XXVII, Figs. 3, 

 A, 8, 9). 



The results of these injections only serve to substantiate 

 Miiller's observations (25). He states that mammals are the 

 only class of vertebrates which possess, in the true sense, a 

 retinal circulation, while in many mammals only a meagre cir- 

 culation is present (horse and rabbit). Denissenko (32) has 

 found an exception to this statement. He describes and 

 pictures retinal blood-vessels in the eel, which penetrate to the 

 outer nuclear layer. This is the only exception which I have 

 so far been able to find. In the sections which I have of 

 the eel's eye, owing to their thickness I have not been able 

 to demonstrate the presence of capillaries beyond the inner 

 boundary of the outer nuclear layer. Fishes and amphibians 

 possess a good circulation in the hyaloid membrane, while birds 



