1813.] 



in the Eyes of Birds, 



173 



cornea about one line within its circumference. In order to 

 demonstrate this muscle it is necessary only to remove the anterior 

 segment of the eye just behind the bony hoop^ and then the 

 pigmentum nigrum being carefully washed away, the iris is to 

 be gently detached from the ciliary circle, and the chbroid coat 

 from the sclerotica. Some delicacy is necessary in performing 

 this part of the operation, for the muscular fibres adhere to the 

 internal surface of the choroid coat, as well as to the bony hoop: 

 if the choroid, therefore, be not slowly and carefully detached, 

 many of the muscular fibres will be separated from the bone, and 

 confounded with the membrane and its pigment. 



When the muscle is thus exposed, its descending fibres will 

 be seen terminated in a well-defined tendinous ring, which 

 advances a little beyond the circumference of the cornea, to 

 which it firmly adheres. The thickness of the muscle, as well 

 as the manner of its insertion, may be most conveniently de- 

 monstrated by cutting the anterior segment of the eye through 

 its diameter. The fibres will then be seen upon that part of the 

 cut edge which corresponds with the bony hoop. To complete 

 the demonstration, a pin, or thin probe, may be passed between 

 the muscle and the sclerotica. The nerves which are seen 

 branching in a singularly beautiful manner through the substance 

 of the muscle,* are derived from the lenticular ganglion, a 

 mere inspection of the attacliments of this muscle will be suffi- 

 cient to suggest its action ; for since the bony hoop, from which 

 the fibres arise, must be considered as a fixed point, the cornea 

 into which they are inserted must be drawn inwards by tlieir 

 contraction ; but the matter admits of demonstration. By 

 means of the galvanic influence, the action of the muscle may 

 be excited in the eye of a turkey a few minutes after the head 

 has been separated from the body, when it may be observed that 

 every contraction of the fibres is attended with a corresponding 

 motion of the cornea; or if the hbres i;e drawn upwards by 

 means of a forceps, the cornea may not only be flattened, but 

 its convexity may be made to respect the i'is. Since, then, it 

 may be demonstrated that this muscle is in its action a depressor 

 of the cornea,, it seems scarcely necessary to add that its ioflaeoce 

 must tend to diminish the convexity of the eye It seems pro- 

 bable, therefore, that the eyes of birds are in the ordinary state 

 possessed of a high refrictive power ; and an eye so constituted 

 seems peculiarly well adaptcfi to the uses of the animal v/ivAq it 

 rests upon the earth, but when it soars into tlie middle regions of 

 the air the rays proceeding from objects below must arrive at the 

 eye in lines v/hich may be considered as parallel ; consequently 

 to form any thiog like a distinct image the focal kogth of Jhe 



« See- Plate Si. 



