17:2 



Description of an Organ 



[March J 



crystalline lens^ tlien (resting upoo the uniformity in naiare's 

 laws) we may be assured that in all animals ihe eye is accommo- 

 dated to the different distances of objects by some change Id its 

 optical constitution, although the precise nature of that change, 

 or the means by which it is eifected in the various classes of 

 animals;, may for ever elude our research. I believe Derham* 

 first entertained the opinion which ascribes to the marsupiitm, or 

 pecten plicatum, in the eyes of birds, the function of regulating 

 the focal distance of the eye. The opinion has^ lately been 

 revived by Sir Everard Home, but it seems liable to insupera!)le 

 objections; for, 1st, the marsupium does not exhibit the least 

 trace of m oscular structure. I have very carefully examined 

 this organ in the ostrich and the eagle : in these great birtts ihe 

 part is of such a size that were it really muscular the fibrous 

 structure could scarcely escape detection, f 



2d. In many birds, as in the turkey, jackdaw, &c. the mar- 

 supium terminates in the substance of the vitreous humour, and 

 has no direct attachment to the lens. J 



3d. in all birds it is situated obliquely with respect to the 

 lens, so that if it acted at all it could merely communicate to 

 the lens a motion of rotation, or remove it a little from the axis 

 of vision : this cannot be better illustrated than by refening to 

 Sir Everard Home's excellent representation of this organ (Phil. 

 Trans. 17^?6). It would spem, then, that we are ignorant not 

 only of the means by wliich the optical constiiiition of the eye is 

 so changed as to be accommodated to the different distances of 

 objects, but that we have not hitherto had satisfactory evidence 

 that any such change takes place. 



Such I believe to be the state of the subject, when in the 

 month of February last an opportunity occurred to me of exa- 

 mining the eye of an eagle, and shortly after that of an ostrich. 

 In those great birds we can view all the peculiarities which 

 distingiush organs of vision in the feathered tribe in general, 

 rendered large and conspicuous. To this circumstance 1 am 

 indebted for the discovery of an organ, which I trust will enable 

 lis to solve a problem in optica] science, wfiich has long occupied 

 the attentioii of some of the most distinguished members of the 

 Royal Society. 



The organ to which I allude is a distinct muscle, which arises 

 from the internal surface of the bony hoop of the sclerotica, and 

 is inserted by a tendinous ring into the internal surface of the 



* Physico-Theol. note to p, 106. 



f See Plate 3d. In the ostrich the marsupium measured of an inchftom the 

 apex to the base, and the longest diameter of the base itself measured -I— of 

 an inch. 



« i Biumenbach's Comparative Anatomy. 



