EYELIDS. MEMBRANA NICTITANS. 735 



piece of mechanism, if we maybe pardoned the expression, met with in 

 the eye of a Bird is destined to regulate the focal distance between the 

 crystalline lens and the sentient surface of the retina, in order to ensure 

 the clearest possible delineation either of near or distant objects. The 

 provision for this purpose is peculiar to the class under our notice, and con- 

 sists of a vascular organ, called the marsupium orpecten, which is lodged 

 in the posterior part of the vitreous humour (fig. 365, a). This organ is 

 composed of folds of a membrane resembling the choroid coat of the eye, 

 and, being in like manner covered with pigment, might easily be mis- 

 taken for a process derived from that tunic, with which, in fact, it has 

 no connexion, being attached to the optic nerve just at the point where 

 it expands into the retina. Its substance seems to be made up of erectile 

 tissue, and it is most copiously supplied with blood derived from an 

 arterial plexus formed by the arteria centralis retince * ; 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 eyeball ; 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. 



(2082.) Birds have three eyelids : an upper and a lower, resembling 

 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 especially appropriated to its motions, so as to sweep over the 

 entire cornea, which it then covers like a curtain. 



(2083.) The upper and the lower eyelids differ but little in their 

 structure 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 eyelashes attached to the palpebral margins ; and secondly, 

 the lower eyelid is the most moveable of the two, and not only contains 

 a distinct arsal cartilage, but is provided with a special depressor muscle, 

 which arises from the bottom of the orbit, like the levator palpebrce 

 superioris of the human subject : the elevator of the upper eyelid and 

 orbicularis palpebrarum are likewise well developed. 



(2084.) The third eyelid, or nictitating membrane, is represented in 

 fig. 366, A, e (the upper and lower eyelids having been divided through 

 the middle, and turned back to display it) : it is necessarily, to a cer- 

 tain extent, transparent ; for birds sometimes look through it, as for 

 instance when the Eagle looks at the sun f : it is therefore of a mem- 

 branous texture, and a most admirable and peculiar muscular appa- 



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

 t Cuyier, Le9ons d'Anat. Comp. torn. ii. p. 431. 



