1872.] Dr. R, J. Lee on the Sense of Sight in Birds. 359 



change in its curvature were advanced as the means by which accommoda- 

 tion was effected. 



The eye of Rhea americana appears to be very similar to that of Struthio 

 camelus, though not quite so large. The globe is of irregular shape, and 

 bulges out both laterally and vertically ; its diameter in the former direc- 

 tion is an inch and two thirds, in the latter an inch and a half, and antero- 

 posteriorly an inch and one third. 



The sclerotic is not particularly thick, and contains but slightly deve- 

 loped osseous structure. The crystalline lens is about half an inch in its 

 lateral diameter, and one third of an inch in its antero-posterior diameter. 

 The ciliary muscle is large and strong, the body thick, and the fibres 

 diminishing in size as they become tendinous near their insertion ; its length 

 is inch. 



The anterior and posterior elastic ligaments are each about T 3 ¥ inch in 

 length, though it is to be understood that their elasticity is so great that 

 they might be stretched to a considerably greater length. 



In the first part of this communication I expressed some doubt regarding 

 the microscopical character of the anterior elastic ligaments ; indeed the 

 term ligament was not applied to them, as they did not possess the same 

 distinct character as the posterior elastic ligament. 



In all the species of birds which have come under my observation, the 

 microscopical character of the last-mentioned structure was the same. In 

 the Rhea the anterior elastic filaments are distinctly composed of the same 

 kind of elastic fibres ; their colour is a light-grey ; they coil up very readily 

 when torn from one another with needles ; they are to some extent covered 

 with fine granular or spongy tissue, which at first conceals their elastic 

 character ; they are continuous and of equal diameter from their origin to 

 their insertion, and are united more closely than in most birds, so that the 

 filamentous character so clearly seen in the Owls is not observed. 



A more complete investigation into the anatomy of this part of the sub- 

 ject allows of the conclusion that the anterior elastic filaments are composed 

 of cellular and elastic tissue combined in different proportions, and that 

 the differences in their strength, elasticity, and appearance depend on the 

 collection of the filaments into fibres of varying sizes, or their approxima- 

 tion so as to form a continuous suspensory band between the iris and the 

 cornea. 



The iris in this bird is not composed entirely of muscular fibres as in 

 many other genera, but is soft and spongy in its general character, and 

 more like the iris in mammalia than in birds. 



As it is desirable to limit myself to those particular structures which are 

 concerned in the accommodation of the eye for distance, deferring for the 

 present certain general conclusions which fresh observations are required 

 to confirm, I shall leave to the consideration of the naturalist the subjoined 

 facts arranged in a tabulated form, and which appear to me to be applicable 

 to the explanation of the habits of the birds by anatomical peculiarities. 



