635 



EYE. 



EYE. 



similarity in position to the eyes of those which undoubtedly possess 

 Tisual powers. The eyes are always very minute however in pro- 

 portion to the bulk of the body, and in no instance do they possess a 

 high type of structure ; their general organisation indeed bears a close 

 resemblance to that which has been described in the eye of the 

 Scorpion. In the Cephalopoda we find the visual organs presenting a 

 much larger size, and attaining a much higher grade of development, 

 in accordance with their greater functional activity in directing the 

 rapid and energetic movements practised by a large proportion of 

 these animals. We here find nearly all the principal parts which are 

 characteristic of the eye of higher animals, namely, a cornea, an 

 anterior chamber filled with an aqueous fluid inclosed in a distinct 

 capsule, a crystalline lens of globular form (as in fishes), a large 

 posterior chamber filled with vitreous humour, a tough fibrous or 

 sclerotic coat, a vascular choroid coat within this covered by 

 black pigment upon its inner surface and retinal expansion. The 

 relations of this last to the optic ganglion however are very peculiar. 

 This ganglion is situated almost close to the back of the eye, and 

 instead of transmitting a single optic nerve as in higher animals, it 

 gives off a multitude of filaments which separately pierce the sclerotic 

 coat, and then form a plexus between this and the choroid, which has 

 been mistaken for the retina. The true retina however is a very thin 

 lamella apparently composed of vesicular nerve-substance, which is 

 found between the pigment and the membrane inclosing the vitreous 

 humour ; but the connection of this with the net-work of nerve-tubes 

 on the outside of the pigmentary layer has not yet been made out. 

 No proper iris exists in the eyes of the Cephalopoda, but its place is 

 supplied by a partial prolongation of the sclerotic coat over the front 

 of the crystalline, a central pupillary aperture being left. The cornea 

 i not, like the true cornea of higher animals, a transparent con- 

 tinuation of the sclerotic coat, but is a modification of the general 

 integument, analogous rather to the external or conjunctival layer of 

 the cornea of Vertebrata : it is remarkable that in some Cephalopoda 

 it should be perforated by an orifice "of considerable size, through 

 which the capsule of the crystalline lens projects into the external 

 medium." (Carpenter, ' Principles of Physiology.') 



The eyeballs of quadrupeds and other Mammalia resemble the 

 human organ in structure, and differ from it, but not essentially, in 

 form. This is not the case with the appendages. One of the most 

 remarkable additions commonly found to the parts we have described 

 is that of a strong retractor muscle in the shape of a hollow cone 

 attached at the apex to the bottom of the orbit, and by the marginal 

 base to the sclerotic, which it embraces, lying under the recti muscles. 

 Its use is to draw back the eye in the orbit, a gesture which gives a 

 very peculiar expression of hollowness to the organ in beasts of prey. 



We subjoin the following account of the eye of the Common Owl 

 hubo), chiefly for the purpose of explaining the pecten and the 

 curious mechanism of the third eyelid, or nictitating membrane, 

 in birds. 



The general shape of the organ represented in the annexed figures 

 resembles a bell. This arises from the disposition of a series of 

 quadrangular bony scales ijiy. 15, a) within the substance of the 

 sclerotic, concave on their outer aspect, and overlapping and accu- 

 rately fitted to each other. The rigidity thus communicated to the 

 external case which contains the fluid media prevents their pressure 

 from distending the eye into a spherical shape. The ciliary body 

 (fiy. 15, b) extends over the whole of this portion of the surface. A 

 curious membrane called the pecten or comb (fig. 15, c), from some 

 resemblance to that implement, projects through the choroid into the 

 vitreous humour, and in some birds is attached to the side of the 

 lens. In the owl it is comparatively short. It resembles a quadran- 

 gular piece of choroid folded backwards and forwards upon itself 

 like the paper of a lady's fan. Of its use little is known. The 

 foramen of Soemmering, described in the account of the human 

 retina, is thought to be a rudiment of the pecten. In birds the retina 

 has generally the yellow colour seen only partially in man round the 

 central spot miscalled a foramen. 



Kig. 15. 



lt'*i i/ontal section of the eye of the Common Owl (Slrix into). 

 <i a, bony platen in the Bclerotic [ b, ciliary body ; c, pecten. 



At the back of the globe there are two muscles which originate 

 from the sclerotic, and are applied to iU curved surface round the 



Fig. 10. 



Head of the same bird. A portion of the bony margin of the orbit bavins 

 been removed, the eyeball is turned forward so as to show the recti and other 

 muscles. 



entrance of the optic nerve. (Fig. 17, a.) The larger represents rather 

 more than half of what if completed would be a broad circular ring. 

 (Fig. 17, b.) It is called the quadratus. Attached by its wider edge 

 near the margin of this part of the sclerotic, its fibres converge to 

 the narrower edge, and terminate in a narrow tendon (fig. 17, c), 

 perforated through its whole length like the hem of an apron. The 

 second smaller muscle, called the pyramidalis from its shape (fig. 17, d), 

 at an opposite part of the circumference. Its fibres converge, and are 

 fixed into a long round tendon (fiys. 17 and 18, e), which passes 

 through the loop or hem (c) of the quadratus, and hence turning over 

 the edge of the broad part of the sclerotic, is continued along the 

 surface of its bell-shaped portion, where it passes through several 

 thread-like loops or pulleys which keep it applied to the concavity, 

 and round a bony point which projects from the surface, and is 

 Fig. 17. Fig. 18. 



Fig. 17. Posterior view of the sclerotic of the same bird, Knowing the 

 muscles of the nictitating membrane, a, optic nerve ; i, quadratus muscle ; 

 c, its looped tendon ; d, pyramidalis muscle ; <*, its tendon having passed 

 through the loop in that of the quadratus turning over the edge of the 

 scleiotic. 



Fill. '8. Lateral view of the same part, e, the tendon of the pyramidalis, 

 attached to the concave part of the sclerotic by tendinous loops, and passing 

 round a prominent bony tubercle, is seen inserted into the nictitating membrane 

 at/. 



attached near the edge of the cornea to the edge of an elastic fold 

 (fig. 18, /) of the conjunctiva, which is called the third eyelid or 

 nictitating (that is, winking) membrane. It will be easily seen 

 by the help of the figures, from this description, that the effect of the 

 simultaneous contraction of the two muscles will be to draw the 

 membrane with great rapidity, making it sweep over the surface of 

 the cornea. It returns by its own elasticity with nearly equal 

 quickness. A bird may be seen to use this mechanism twenty times 

 in a minute ; in fact, as often as it may be necessary to cleanse the 

 surface of the eye. The colour of the membrane is milky ; and it is 

 seen to pass from the upper and inner to the outer and lower corner 

 of the eye with the speed for which the act of winking is proverbial. 

 There is a rudiment of this third eyelid in the human organ. It is a 

 small crescentic fold of conjunctiva situated at the inner canthus 

 behind the caruncle. (Fig. 13, /.) The haw is also a rudiment of 

 it, in the eyes of quadrupeds ; it is occasionally forced out by the 

 pressure of the globe against the nasal side of the orbit, being 

 unprovided with muscles. 



Seat of Vision. The retina in one sense is not the seat of vision. 

 It is necessary to the perception that the impression of light should 

 be received on another part not endowed with sensibility, namely 

 the surface of the choroid ; and that the vibration or other effect thus 

 impressed should be transferred to the retina in front of that surface ; 

 for where the choroid is deficient at the entrance of the nerve, there 

 is no perception of light. This may be easily shown by a very 

 common and conclusive experiment. If two discs of white paper ba 

 fixed upon a wall at the distance of two feet from each other, and an 

 observer, having closed one eye (the left), continues to gaze atten- 

 tively at the left-hand disc, at the same time slowly retreating from 

 the wall, he will for a time continue to see them both ; the rays from 

 the right-hand object entering of course laterally, and impinging upon 



