668 PHILOSOPHICAL TBANSACTION9. [anNO 179(5 



cular elastic ligament firmly attached to the anterior edge of the bony rim, and 

 lying on the outside of the basis of the cornea; there is a similar ligament in 

 other birds, but less conspicuous. This bony rim in the eyes of birds is parti- 

 cularly noticed by Haller; specimens of it, whole and in separate parts, are pre- 

 served in Mr. Hunter's collection ; it has been also described by Mr. Smith, in 

 a paper read before this Society: I shall therefore not dwell longer on its struc- 

 ture, as it is not to my present purpose to take further notice of it than to explain 

 its use respecting the adjustment of the eye, the subject of the present lecture. 



The straight muscles of the eye in birds arise from the bottom of the bony 

 orbit, as in the quadruped, and are firmly attachetl to the posterior edge of the 

 bony rim just described; they are 4 in number. The ciliary processes are larger 

 and longer in birds, than in other animals whose eyes are of the same size; 

 they are evidently continued from the choroide coat, and adhere firmly to the 

 capsula of the crystalline lens. In the eyes of birds there is a substance which 

 is peculiar to that class of animals, called the marsupium. It is a process com- 

 posed of a corrugated vascular membrane attached to the centre of the retina, 

 where the optic nerve terminates. Its origin is in a straight line, extending from 

 the termination of the optic nerve to the lower part of the eye; in the turkey ^ 

 of an inch in length, and connected with the bottom of the eye by an elastic 

 ligament about V-o ^^ ^" ^'^^^ thick. The number of folds of which it is com- 

 posed varies in different birds, from 5 to 15, or more; they are all of the same 

 length, which in the turkey is about + of an inch ; they are covered with the 

 nigrum pigmentum, and are attached anteriorly to the capsula of the crystalline 

 lens, either immediately, as in the goose, or by intermediate membrane, as in 

 the turkey. 



The structure of the marsupium is very similar to that of the ciliary pro- 

 cesses, but stronger in all its parts, and like them it has a connection with the 

 crystalline lens. The connection between the marsupium and lens, in a natural 

 state of the parts, is from its transparency invisible; but in the goose and cas- 

 suary, where the marsupium extends to the capsula of the lens, if the parts are 

 coagulated in spirits, it becomes very apparent, and in these birds such a con 

 nection is generally allowed. In other birds, it is doubted by some, and denied 

 by others, who have written on the subject. Haller has taken some pains on 

 this point; he found, that by pulling the marsupium the motion was communi- 

 cated to the lens, but he was unable to make out the mode of union; and all his 

 attempts to coagulate the cells of the vitreous humour were unsuccessful ; he 

 says, no spirits can produce such a change. I have found however, that after 

 the eye has remained a few days in rectified spirits, the medium between the 

 marsupium and lens is coagulated and rendered visible. By this means I have 

 detected it in the turkey's eye; it is connected to the whole anterior extremity of 



