612 AVES BIRDS. 



lodged in the posterior part of the vitreous humour (Jig- 279 a). 

 This organ is composed of folds of a membrane resembling 

 the choroid coat of the eye, and, Fig. 279. 



being in like manner covered with 

 pigment, might easily be mistaken 

 for a process derived from that tunic ; 

 with which, in fact, it has no connec- 

 tion, being attached to the optic 

 nerve just at the point where it ex- 

 pands into the retina. Its substance 

 seems to be made up of erectile tis- 

 sue, and it is most copiously supplied 

 with blood derived from an arterial 

 plexus formed by the arteria centralis retinae ;* so that there is 

 little doubt that, being like the iris endowed with an involuntary 

 power of dilatation and contraction, as it enlarges from the injection 

 of blood, it distends the chamber of the vitreous humour, and 

 pushes forward the lens ; while, as it again collapses, the crystalline 

 is allowed to approach nearer to the retina, and thus the focus of 

 the eye is adjusted upon the same principle as that of a telescope. 

 Four recti and two obliqui muscles preside over the movements 

 of the eye-ball ; but, as in the Reptilia, the superior oblique arises 

 from the anterior part of the orbit, as well as the obliquus inferior, 

 and its tendon is not reflected over a trochlea. 



(681.) Birds have three eye-lids : an upper and a lower, resem- 

 bling those of mammalia ; and a third, which, when unemployed, 

 is concealed in the inner canthus of the eye, but can be drawn 

 down vertically by muscles specially appropriated to its motions, 

 so as to sweep over the entire cornea, which it then covers like a 

 curtain. 



The upper and the lower eye-lids differ but little in their struc- 

 ture from those of Man ; nevertheless, a few trivial circumstances 

 are worthy of the notice of the student. In the first place, there 

 are seldom any eye-lashes attached to the palpebral margins ; and, 

 secondly, the lower eye-lid is the most moveable of the two, and 

 not only contains a distinct tarsal cartilage, but is provided with a 

 special depressor muscle, which arises from the bottom of the orbit 

 like the levator palpebra superioris of the human subject : the ele- 

 vator of the upper eye-lid, and orbicularis palpebrarum, are like- 

 wise well developed. 



* Vide Barkow, in Meckel's Archiven, Band xii". 



