ANIMAL I'll 



33 



AN I M A L PHYSIOLOGY. L 



THK KVB. 



,.) \* thi> instrument by which the mind booomos ac- 



iiiuiinti'd with ,'itiTiial and distant objects by means of tho litfht, 



is inio i.f tho moat nubile and delicate forces in nature, 



iiiul ni-i'iU :i <-oi ivupondingly delicate and complicated organ to 



ui>|>ivri:it ita effects. 



Without inquiring into the nature of light, it i Bufficiont for 

 our Bubjoot that wo know 

 Bomo of its constant i[ii:ilitii-i, 

 or lawn, as they have boon 

 called. 



In ita simplest condition 

 light travels in titruitfht linos 

 in all directions, from its 

 source; hoiioo, when wo M'O 

 a luminous body, wo know the 

 direction in which it lies, be- 

 cause it must liu in tho lino of 

 the ray which roaches us. 



When a ray of Tight thus 

 travelling in a straight lino 

 strikes upon the surface of any 

 object, it is affected in some 

 of the following ways accord- 

 ing to tho nature of the object 

 and of its surface : 



1st. It may be destroyed, as 

 far as visual effects are con- 

 cerned, partially or wholly. 



2nd. It may penetrate the 

 substance of the body, being 

 more or less bent as it traverses 

 the surface. This occurs when 

 the body is transparent. 



3rd. It may glance off and 

 pursue a different direction 

 outside the object upon which 

 it strikes. 



The first effect is called ab- 

 sorption ; the second, refrac- 

 tion ; and the third, reflection. 



Reflected light concerns us 

 most. The eye occupies itself 

 with reflected rays. If light were incapable of being reflected, 



the sun would appear as a sharply- defined dazzling orb in a 

 pitch-dark universe, and eyes would be of no use ; for though 

 poets tell us so, not even the eagle spends its time in so profit- 

 less and injurious an 

 employment as gaz- 

 ing on tho sun. 



Now, as reflected 

 light travels in 

 straight lines from 

 the object upon 

 which it is reflected, 

 it is to the eye, in 

 all respects, the 

 dame as though that 

 object wore itself 

 luminous. As light 



proceeds from all parts of an object, and travels in straight 

 lines, we have only to let the rays fall upon some surface which 

 shall receive them without derangement, to get an image which 

 will give the colour, form, and, by a little inferential reasoning, 

 the size and distance of the object. 



The first requisite in an eye, then, is a sentient mirror, which 

 shall receive the images of objects and feel them. 



This mirror must be of moderate and portable size, and well 

 under control, so that it can be turned about. 



All mirrors are perishable and delicate articles, liable to frao- 



problem of how to make a serviceable eye is a diffi- 

 cult ono. 



The analogy of the mirror, however, most not lead the reader 

 to suppose that a plane surface, sensitive to light, would be con- 

 scious of distinct images, or that it would see object* as we, 

 by the .. > ye, see them reflected on its surface. For 



distinct vision, it is necessary that many divergent rays proceed- 

 ing from each point in an object should be collected together 

 again in a point, and that point must lie exactly on the retina* 

 or sentient mirror. Thus, the instrument known as a *~\ 



which has a lens set into the 

 aide of a box, and a surface at 

 the other side to receive the 

 image, is a; 

 for an eye. 



We will now describe the 

 structure of one of the most 

 perfect instruments for taking 

 note of tho impression pro- 

 duced by light with which we 

 are acquainted the human 

 eye. 



The human eye is globular; 

 differing, however, from a per- 

 fect sphere in some slight but 

 important particulars. The 

 thick, tough capsule, which 

 maintains the shape of the eye, 

 and contains all the other parts 

 necessary to perfect vision, ia 

 about one inch from front to 

 back, and a little more from 

 eide to side and from top to 

 bottom. This is called the 

 sclerotic, or hard coat of the 

 eye. This hard coat, which 

 forms the eyeball, differs from 

 a true sphere also, in that ita 

 front part, occupying about 

 one-sixth of its circumference 



sclerotic or hard coat of the eye; b, choroid; c, retina or nervous (in section), bulges forward far 

 mirror; d, membrane holding the vitreous humour; , vitreous more than it would do if it 

 humour ; /, cornea ; g, aqueous chamber and humour ; 7i, crystalline wero only a part of 

 lens; i i, iris; fc fc, ligament to hold lens; I, meibomian glands; m m, J.IQ.VJQ 

 muscles to wield the eye; , muscle to lift the eye-lid. gj fcj ^^ ^ ^^ ^ 



for while it is equally strong 



and tongh, and even harder, it is purely transparent, while the 

 rest of the eyeball is opaque and white. This front clear por- 

 tion, which is let into the hinder part as a bay-window is put 

 into the vail of a room, or as an old-fashioned watch-glass is 



set into the rim of 

 the watch-case, ia 

 called the cornea, or 

 horny structure. Its 

 greater projection or 

 convexity is not a 

 matter of accident, 

 but highly import- 

 ant, for if it were 

 not so, no near ob- 

 ject could be 

 distinctly. 



Lining the 



surface of tho sclerotic is a thin membrane, which supports 

 in its outer layers the larger arteries and veins which carry 

 the blood to and from the front and inter parts of the eye, 

 while it has on its inner surface a very thin pavement of 

 flat, six-sided cells; each cell filled with black grains. The 



1. VERTICAL SECTION OF THE HUMAN KYE IN ITS SOCKET. 



2. DIAGRAM SHOWING HOW OBJECTS ARE IMPRINTED ON THE RETINA. 



grains, and even the cells which contain them, are so small and 

 so closely sot as to form what appears to any but a high mag- 

 nifying power, a continuous thin black sheet, perfectly opaque. 

 Thia membrane papers the inside of the eye as far forward as 

 i the place where the sclerotic joins the cornea, and is there oon- 





ture ; but when we conceive of a mirror whose surface and nected firmly with this outer jacket by a strong ligament and 

 backing, and even its very frame, must be made not of hard muscle. Before it reaches this point, however, it is puckered 

 glass, imperishable quicksilver, and durable wood, but of soft I into somewhat irregular fore-and-aft folds. Beyond this point 

 renewable tissues, and think how indispensable it is that it j tho choroid, as this membrane is called, is continued as a freely, 

 should be protected and kept in a state of repair, we must admit , hanging curtain, shaped like a quoit, that is, round and opaque, 



VAT . T. 8 



