1872.] 



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



355 



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 pos- 

 terior 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 composed 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 struc- 

 ture — 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 side of, the line of insertion of the pos- 

 terior elastic ligament. The breadth of the latter structure is about one 

 eighth of an inch, while the length of the anterior elastic filaments is 

 nearly the same. Thus, passing from before backwards, we have the an- 

 terior elastic filaments, the body of the ciliary muscle, its long delicate 

 tendinous portion, and lastly the posterior elastic ligament. To exhibit 

 the structures satisfactorily, the best plan is to make a section of the cho- 

 roid and sclerotic of one sixteenth of an inch in thickness, and after fixing 

 the two ends of the section on a layer of cork with needles, to dissect the 

 muscle under water or alcohol — a very simple process if a magnifying-glass 

 of an inch focus is employed. It is only necessary to draw the iris gently 

 away from the sclerotic so as to extend the anterior elastic filaments, fixing 

 it with a needle, and then to do the same with the choroid, taking care to 

 hold that membrane at a point posterior to the line of insertion of the pos- 

 terior elastic ligament. 



The length of the ciliary muscle is about three eighths of an inch. I have 

 attempted to preserve sections made in this way in Canada balsam, but 



