47* » Stiudiiri of the Eyft of Birds. 



neclcft with the cryftalline Ions at the one extremity, and at the other witli the bottom of-tlie 

 eye. By experiments on this membrane he afcertained, tliat it is cspable of mufcular con- 

 tradiion : for, when the marfupium and cryllaUine lens of a goofe's eye were expofed im- 

 mediately afier death, and the lens was pulhcd forwards fo as to elongate that membrane 

 from J,;ths to i^ths of an inch, it repeatedly recovered its ciiginal dimenfion when the 

 prelTure was taken off. But when the parts had been left until all remains of life were 

 gone, and the tenfion was then made as before to |^ths of an inch, it contracted only to 

 ,*oths; whence it was concluded, that the difference between the contratlions was the. 

 effect of mufcular a»3ion. It was obfervable, however, that in fo minute a quantity it was 

 eafy to be deceived. Another experiment was therefore inftitutcd upon the well-known 

 faft, that in the a£l of dying, the mufcles are found to contraft to their utmoft where there 

 is no refinance to prevent fuch aiTlion ; and that this is alfo found to take place in the 

 greateft degree when the animal is killed by any violence committed upon the brain or fpinal 

 marrow. The cryftalline lens of a turkey's eye was extracted, and immediately afterwards 

 the turkey was killed by wounding the fpinal marrow. The two eyes were tnken out and 

 put into fpirits. In the one the marfupium had nothing to prevent its contracting to the 

 utmoft ; while in the other the lens being in its natural fituation could not allow of any un- 

 ufual contraction. Some days afterwards the two eyes were examined : in ihe perfedl 

 eve the marfupium meafurcd ,*.ths of an inch, and its different folds were femi-tranfpa- 

 rcnt ; in the imperfect eye the marfupium meafured ^^ ths of an "ich, and the folds were 

 mucli more opake. Here then was a difference of 4-J-^ of an inch in the length of the two 

 marfupiums : which could arife from no other caufe than the one having contracted fo much 

 more than the other, which contraction mull be conlidered to be mufcular. 



On a review of the peculiarities in the eyes of birds the author infers, that they tend to fa- 

 cilitate the lengthening of the axis of vifion, and increafing the convexity of the cornea. 

 The bony rim to wliich tlie mufcles are attached, confines their effeCt to the broadeft part 

 of the eye ; and as their aCtion throws the cornea forward, the anterior rim of the bony 

 edge yields to adapt itfelf to that change. The ciliary proceil'es are long, and admit of the 

 lens t)eing moved forward during the adjuftment for very near objeifts, which is performed 

 with more facility than in other animals, while their aClion ferves to bring it back to its 

 place. The marfupium ferves alfo to draw it backwards, and, by fuftaining part of the pref- 

 fure from behind, renders the cornea flatter, while the anterior edge of the bony rim is 

 adapted to it in this Hate. It may be faid that no fuch great change is neceffary for vifion 

 with parallel rays; but Mr. Home remarks, that where vifion is to be very dillinCl, a cer- 

 tain nicety of adjuftment becomes neceffary, and the aCtion of the marfupium is probably- 

 intended for that purpofe. 



Tlie fubjeCl of vifion in birds is concluded by an obfervation, that one of the mod beau- 

 tiful illuftrations of the combination of mufcular andclalHc fubftances is feen in the motion 

 of the nictitating membrane. This membrane is tlaftic, and is connected by means of a 

 tendon with two mufcles fituat d upon the pollerior part of the eye-ball. The action of 

 thefe mufcles brings the membrane ever the cornea; and the inftant they ceafe to contraCl, 

 the elaftlcity of the membrane draws it back ag;iin. 



The eyes of fiflves have feveral peculiarities, and in many rcfpeCls their flruClure differs 

 ^om that which is obferved in the quadruped and bird. 



The 



