189(3.] Acute Vision in the Vertebrates. 31 



With birds, the presence of a fovea seems to be the rule. In 

 fact, the domestic chicken is thus far the only exception. 

 Many birds have a fovea and band-like area, while some have 

 two fovese and a band-like area connecting them. 



In reptiles, the number of species provided with fovea or 

 simple area are more nearly equal, while with amphibians 

 and fishes, the area has frequently not been seen, and the fovea 

 is only seldom observed. 



The area centralis varies greatly in form and extent in dif- 

 ferent animals. It varies from the round form of small extent 

 found in the cat and the weasel to the band-like form found in 

 the horse, sheep, rabbit, frog, etc., which extends horizontally 

 across the retina. 



In the case of the fovea we also find a variety of forms and 

 positions. In some animals it is situated on the nasal side of 

 the entrance of the optic nerve {fovea na*ali»), while in others 

 it is on the temporal side (fovea temporalis). According to 

 Muller, 6 in the former case we have monocular vision, while in 

 the latter we have binocular vision. In form it varies from a 

 mere dot-like impression, as in some lizards, to a well marked 

 funnel-like pit in most birds, especially crow, bluejay, robin, 

 etc., and to a trough-like depression in the crocodile which 

 extends horizontally across the retina. Two fovea? have been 

 found in some birds, as in swallows and terns, in which case 

 the fovea nasalis is very near the centre of the retina, and lias 

 to do with single vision. It is also larger and deeper than the 

 fovea temporalis, which is situated near the ora serrata and 

 functions in double vision. According to Chievitz, 7 the tern 

 has not only two fovre, but a trough-like fovea connecting them, 

 and the goose, duck and gull have a round fovea and a band- 

 like area. 



A great difference exists in the different vertebrates when 

 their ability for acuteness of sight is considered. It varies from 

 the most perfect sight found in man (and possibly in birds 



6 II Mullcr. 1'f-l.n.T da- Yorhand-.-in zweier Fovea in der Netzhaut Vk-\-r 

 Vogelaugen— 7. 3ept, 1863, p. 438-440; or 

 Anatomie und Physiologie des Auges p. 139, 142-143. 



7 J. H. Chievitz I la- York \ - ivtina Uchiv. t". 

 Anat. u. Entwick., 1891, p. 324. 



