Mr. R. J. Lee on the Sense of Sight in Birds. 147 



in. 



Diameter of cornea -j*^ 



Lateral diameter of eye 1 



Antero-posterior diameter of eye I 



Length of ciliary muscle -^ 



Length of posterior elastic ligament "I ^ 



Length of anterior elastic filament J i^ 



In order to ascertain the mechanical effect produced by the ciliary 

 muscle, the simple experiment may be performed of applying traction, 

 by means of a pair of forceps, on the choroid, the dissection being 

 arranged and fixed as I have described. It will readily be seen that 

 the elastic ligament acts in direct opposition to the muscle, and in 

 the living eye has the power of restoring the parts to the condition 

 of rest. 



The ciliary ntiuscle is composed of striated fibre of very distinct 

 character. It varies, as is seen in the three examples described, in 

 length and amount of muscular tissue. The tendon in the Owl is 

 long and the body of the muscle short ; but in the other species, as 

 in most birds, the muscular fibres extend to a great length, if not 

 entirely from the origin to the insertion of the muscle. These minute 

 differences should be pointed out in detail in the case of each species 

 of bird. 



The elastic ligament is composed of very delicate elastic tissue, the 

 microscopical character of which is well defined. 



On the peculiar nature of the anterior elastic filaments I beg to 

 postpone any decided opinion. 



With regard to the nerves which supply the ciliary muscle and 

 the iris, I have no particular remarks to offer, as the description 

 which I gave some years ago of the ganglia and plexuses on the 

 ciliary nerves in the eye of the Pheasant will apply generally to all 

 birds. Whether the contraction of the iris and the accommodation 

 of the sight be voluntary or involuntary actions on the part of birds 

 we cannot say positively ; 1 am inclined to believe that the latter is 

 the case. 



For the sake of convenience, and to render any further researches 

 on the dimensions of the different parts of the eye in other species 

 of birds symmetrical with those contained in this communication, I 

 have arranged the principal facts in a tabular form (see p. 148). 



From this Table we may draw the following conclusions: — 

 that in the Eagle Owl the range of vision is small, the power of 

 accommodation very rapid ; in the Vulture range of vision is great, 

 the power of accommodation considerable, but slower than in the 

 Owl ; in the Buzzard the range of vision is greater still, and the power 

 of accommodation capable of being readily and extensively exercised. 



These conclusions, I think, will be found to accord with the obser- 

 vations of those who have had opportunities of making themselves 

 acquainted with the habits of the birds during life. 



It has been usual for those who have devoted much attention to 

 the physiology of vision to propose some original and independent 



