Mr. R. J. Lee ow the Sense of Sight in Birds. 145 



The shape of the lens does not appear to be altered by the action 

 of alcohol ; but the size is diminished, and the measurements just 

 stated are less than they would be found to be if the lens had been 

 perfectly fresh. 



The eye is first to be divided into halves by cutting through 

 the sclerotic, choroid, cornea, and iris. We may regard the sclerotic 

 as a hollow case enclosing a sphere, of which the choroid is the 

 proper covering, and which sphere is attached to its case by tissues 

 of highly elastic and muscular properties, by which a certain amount 

 of movement is capable of being effected in the parts on which the 

 formation of the image depends. It is to be observed, however, that the 

 posterior surface of the choroid is kept in close apposition to the 

 inner and posterior surface of the sclerotic, so that movement of the 

 anterior parts is not communicated to that part on which the optic 

 nerve is expanded. In the eye of the Eagle Owl these conditions 

 are obtained in the following manner. 



The whole of the posterior surface of the choroid which corresponds 

 to the optic disk is kept in close apposition to the sclerotic by the 

 direct attachment of the circumference of the part immediately 

 beneath the margin of the retina ; it is also fixed where the nerve 

 passes through the sclerotic, while delicate fibres from the choroid 

 keep it in its position at other points. 



The anterior part of the choroid, on the contrary, is not in contact 

 with the sclerotic, as the ciliary muscle and the structure I have termed 

 the posterior elastic ligament intervene. 



This division of the choroid is not artificial, but is clearly defined 

 by a difference of structure. The posterior part is but slightly 

 vascular, is not elastic, is of considerable tenuity, and has greater 

 resemblance in its general characters to the choroid of fish than to 

 that of the mammalia. 



The anterior portion is covered on its internal surface by the ciliary 

 processes, which extend to the angle of curvature of the posterior 

 part of the eye. The tissue of this part of the choroid is of peculiar 

 character ; it is dense, strong, and inelastic, and appears to be com- 

 posed of delicate fibrous tissue. The combination of these characters 

 enables it to preserve its symmetrical shape, and ensure to some 

 degree the preservation of the structures within it. It possesses a 

 rigidity which may be compared to that of ordinary writing-paper, 

 and is of about the same thickness. The anterior part of the choroid 

 is attached to the sclerotic by another structure — a system of fine 

 elastic fibres which pass from the corneal margin of the sclerotic to 

 the line of union between the iris and the choroid, and for which I 

 proposed the name of anterior elastic filaments. Between the an- 

 terior elastic filaments and the posterior elastic ligament (a distance 

 in the eye of the Eagle Owl of nearly five eighths of an inch) is 

 interposed the ciliary muscle. The body of the muscle is attached 

 to the line of union of the sclerotic and cornea, so that it may be said 

 to arise from the anterior angle of curvature. The greater part of the 

 posterior portion of the muscle is of delicate tendinous structure ; its 

 line of insertion into the choroid is the same as, but on the opposite 



