PATTERN-DISCRIMINATION IN VERTEBRATES 341 



of separate papers as individual problems are completed. In a 

 later communication I hope to discuss some results obtained by 

 other students in the light of results of the present work. 



As is well known, the dog's eye is very imperfect in some 

 respects. The imperfections are not so much those inherent in 

 his eye as a refractive system as those of lack of development 

 and imperfect functioning of other parts. According to Slonaker 2 , 

 the dog's eye possesses no fovea, and even the " sensitive area ' 

 is not well defined. Slonaker was unable to demonstrate a 

 round " sensitive area " slightly temporal to the nerve entrance, 

 which Chievitz described; and he mentions the disposition of 

 the blood-vessels as indicative that the area is band-like. The 

 pupillary opening is always large; the response to light is fairly 

 quick but not strong; movements of rotation are very slight. 

 The writer has demonstrated convergence in several individuals, 

 however. 



The monkey's eye, like those of the other primates below man, 

 possesses a well defined, round " sensitive area," and a well 

 developed fovea centralis (slightly temporal to the nerve-entrance) 

 of medium depth, as measured in the number of layers of cells. 

 The pupillary opening is small in strong light ; responses to light 

 are quick and strong ; pupillary changes apparently accompanying 

 changes of accommodation are marked. There is unmistakable 

 convergence; rotatory movements are coordinate, but are more 

 limited in range than are those of the human eye. This is 

 probably due to the fact that the orbit is deep and the eyes are 

 set rather deeply in it. 



The chick is said by Slonaker to have so shallow a depression 

 as to be very doubtful; he found only a slight thickening in 

 the region where Chievitz reports an area nasalis and a question- 

 able fovea nasalis. Birds in general are characterized by the 

 presence of a fovea, and in some species, such as the tern and the 

 sparrow-hawk, there are two; one temporal and one nasal. 

 The temporal fovea may be regarded as a modification of the 

 fovea centralis, considerably displaced. Due to the position 

 of the eyes in the head, some birds which possess it cannot make 

 close convergence. By dilating the pupil it is possible to image 

 an object lying directly in front of the bird, on to both temporal 



s Slonaker, J. R., "A Comparative Study of the Area of Acute Vision in Verte- 

 brates," Journal of Morphology, vol. 13, 1897, pp. 448 ff. 



