Mr. R. J. Lee on the Sense of Sight in Birds. 149 



were made very soon after the publication of this new anatomical fact, 

 I am gratified in having the opportunity of pointing out the cause of 

 the discrepancies in opinion which have continued to the present 

 time. 



The description which Crampton has given is correct so far as it 

 goes, but it was limited to that part of the ciliary muscle which forms 

 the thickest portion of it — that is to say, the dense part which lies 

 closest to the margin of the cornea. The tendon of the muscle and 

 its insertion into the choroid were not observed by Crampton, and 

 the structure termed the posterior elastic hgament was overlooked. 

 It can thus be explained how it was that the deflection of the margin 

 of the cornea and consequent change in its curvature were advanced 

 as the means by which accommodation was eifected. 



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

 Struthio camelus, though not quite so large. The globe is of irre- 

 gular shape, and bulges out both laterally and vertically ; its diameter 

 in the former direction 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 

 developed 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 cihary 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 -^ 

 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 liga- 

 ments ; 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 com- 

 posed 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 cha- 

 racter so clearly seen in the Owls is not observed. 



A more complete investigation into the anatomy of this part of 

 the subject 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 fi.laments into fibres of 

 varying sizes, or their approximation so as to form a continuous sus- 

 pensory band between the iris and the coruea. 



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

 Ann. & Mag. N. Hist. Ser.4. VoI.tl. 12 



