No. 3-] ACUTE VISION IN VERTEBRATES. 477 



(PL XXVII, Fig. 5). In the representation the retinal vessels 

 were not injected, and the smaller branches could not be accu- 

 rately made out. In cross sections only a slight thickening of 

 the retina is noticed, and the lower edge of the indicated region 

 corresponds with the lower margin of the tapetum. 



Dog (Canis familiaris). 



The retinal blood-vessels (PL XXVII, Fig. 3) indicate the 

 presence of a band-like area. Again the finer and more numer- 

 ous branches radiating toward a common spot on the temporal 

 side of the nerve entrance points to the presence of a round 

 area. Neither are visible to the naked eye, but Chievitz (31, b) 

 has described the presence of a round area in this latter position. 

 I have not succeeded in demonstrating it. 



Birds. 



Birds are characterized by the presence of a fovea, although 

 a few cases are very doubtful (hen and guinea hen). Chievitz 

 says (31, c) that at least a round area is always present which 

 regularly possesses a fovea, sometimes very clearly seen, and 

 in other cases so shallow as to be very doubtful (duck and 

 hen). 



Where but a single fovea is present the position and form 

 are so similar, as shown in the tabulation, that a large number 

 may be described together. As a rule it is situated about the 

 center of the retina, a short distance above and toward the nasal 

 side of the optic nerve entrance. The nerve entrance is always 

 more or less obscured from view by the pecten, which extends 

 obliquely from the point of entrance downward and forward, so 

 that a line joining the fovea and nerve entrance forms about a 

 right angle with the pecten (PL XXVII, Figs. 17, 23). The 

 fovea, with but few exceptions which will be described separately, 

 is surrounded by a simple round area more or less sharply 

 marked off from the surrounding retina. The fovea varies 

 considerably in depth. In the tabulation I have classified them 

 as deep, medium, and shallow. 



Most birds possess a deep and well-defined fovea, as seen in 

 the following: 



