This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
The obvious consequence of this figure of the globe of the eye is to allow room for a greater proportion of aqueous fluid, and for the removal of the crystalline lens from the seat of sensation, and thus produce a greater convergence of the rays of light, by which the animal is enabled to discern the objects placed near it, and to see with a weaker light; and hence Owls, which require this sort of vision so much, possess the structure fitted to effect it in so remarkable a degree.
Fig. 364. Eye of the Owl.
* Rees's Cyclopaedia, art. Birds.
(2080). But it is evident that, in order to retain this conical shape of the eyeball, some further mechanical arrangements are necessary, which in the spherical form of the human eye are not requisite. In Fishes, where the eyeball is constructed upon entirely opposite principles, being compressed anteriorly, cartilaginous supports are found imbedded in the sclerotic tunic, which in some cases is absolutely ossified into a bony cup. In many Reptiles the same end is obtained by placing a circle of bony plates around the cornea; and this latter plan is again adopted in Birds, to maintain their eyes in a shape precisely the converse of the former. In the Owls these ossicles are most largely developed; in such birds they form a broad zone (fig. 364), extending from the margin of the cornea, embracing the anterior conical portion of the eye, and imbedded between two fibrous layers of the sclerotic. The figure which is thus given to the eye, from the increased space obtained, is evidently calculated to allow the humours, forming the refracting media whereby the rays of light are brought to a focus upon the retina, to become materially changed in shape; and both the convexity of the cornea and the position of the lens may thus be altered so as to adjust them in correspondence with the distance at which an object is viewed. The cornea is rendered more convex, and the shape of the aqueous humour consequently adapted to examine objects close at hand, by the simple action of the muscles that move the eyeball: for these, seeing that the edges of the pieces composing the bony circle overlap each other so as to be slightly moveable, as they compress the globe of the eye, cause the protrusion of the aqueous humour, and the cornea becomes prominent; or if the bird surveys things that are remote, the cornea recedes and becomes flattened, - an effect caused by the recession of the aqueous humour, and, as some authors assert*, by muscular fibres disposed around the circumference of the cornea, and attached to its inner layer, which draw back the cornea in a manner analogous to the action of the muscles of the diaphragm upon its tendinous centre.
(2081). But the most beautiful 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 consists of a vascular organ, called the marsupium or pecten, 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 mistaken 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 retinas *; 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.
Fig. 365. Section of the eye of an Owl.
* Vide Cyclop, of Anat. and Phys. p. 304.
(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 palpebral 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 certain extent, transparent; for birds sometimes look through it, as for instance when the Eagle looks at the sun 1: it is therefore of a membranous texture, and a most admirable and peculiar muscular apparatus is given, by which its movements are effected. This is placed at the back of the eyeball, and may easily be displayed by turning aside the recti and obliqui muscles, as in fig. 366, b. Two muscles are then perceived arising from the globe of the eye, taking their origin from the outside of the sclerotic coat: one of these (c), named the quadratics membrance nictitantis, arising from near the upper aspect of the eye, descends towards the optic nerve; but instead of being inserted into anything, as muscles usually are, it terminates in a most remarkable manner, ending in a tendinous sheath or pully, through which the tendon of the next muscle passes as it winds around the optic nerve. The second muscle (d), called the pyramidalis memb. nictitantis, arises from the inner aspect of the eyeball; and its fibres are collected into a long slender tendon, which, as it turns round the optic nerve, passes through the tendinous sheath formed by the quadratus, as a rope through a pully, and then is continued in a cellular sheath formed by the sclerotic, underneath the eye, to the lower angle of the third eyelid, into which it is inserted. The reader will at once perceive how beautifully these two muscles, acting simulta-neouslv, cause the nicti-tating membrane to sweep over the cornea, which returns again into the inner canthus of the eye by its own elasticity.