Book VII. PHYSIOLOGY OF THE HORSE. 901 



enjoys the greatest motion. United, they form an admirable curtain to defend the eye 

 from dust, insects, and the light during sleep ; and are moved by two appropriate mus- 

 cles. Attached to the edge of each is a cartilaginous rim, called the tarsus. The cilia, 

 or eyelashes, are not, as in the human, above and below ; the upper lid only is fur- 

 nished with hairs, and which are not placed in one, but in several smaller rows. The 

 horse has no supercilia or eyebrows, unless we reckon as such the few long hairs over 

 the orbits. The lachrymal gland is a body lodged within the upper part of the orbit, 

 and is furnished with five or six excretory ducts, which secrete the lachrymas or tears 

 to lubricate the surface of the globe. The superfluous tears are carried off by two 

 openings at the inner angle, called puncta lachrymalia, by which means the tears are 

 at once carried into the nose, and not as in man first into a lachrymal sac. The 

 caruncula lachrymalia is a small black substance in view at the inner canthus, whose 

 office appears to be to direct the tears aright in this course. The haw, or nictating 

 membrane, is an important part, seen when the eye is drawn inwards, but which is at 

 all other times hidden within the fatty matter surrounding the globe of the eye. Though 

 called a membrane it is cartilaginous, and when the eye is forcibly withdrawn into 

 the socket, it is pressed out from the inne angle, and passes completely over the 

 surface of the globe, to which its shape is adapted. A moderate pressure only, shows 

 about half of it; and it is thus seen in tetanus or stag-evil, by the action of the retractor 

 muscle ; and under inflammation of the eye it also becomes visible, which has led 

 ignorant farriers to cut it off, under a suspicion that it formed one cause of the 

 disease. The use of this nictating membrane cannot be for a moment dubious. It is 

 denied to man and to monkies, because they, havmg hands, can with their fingers re- 

 move dust and dirt from the eyes ; but to the horse and most other quadrupeds, it is essen- 

 tially necessary for these purposes. - ,, 



5698. A diagram of the eye {fig. 617.) dis- ^.^-/f^^^^^^iS. .'^ 

 plays the transparent cornea in front of V jr\\ A \^*''^ 



the eye (a), the crystalline lens (6), its "..,^ / Bv\ / \ - ^^n\ jt 



posterior convexity (c), its anterior con- /f^ ''n~^ U I \ ^ *^- 



vexity (d), the iris, or curtain (e, /), the ~ / "^'-... /-- 1_^ ;I1 



anterior chamber occupied by the aqueous ,y I >y '" L_!^rrr:r:^79iJH. ^ 



humor of the pupil {g), the posterior ^ I ^ ...-'''^ V^^^^^-^^'^'^m 



chamber filled with the vitreous humor \ ^ >'*!. 7 -, 'ig. 1 



ih h), the retina (z), the choroid coat {k), It/ ..\-''' J^\ '^ f/f 



the sclerotic coat (^), and optic nerve (?w), ?. ,.--"" \^'' jil \ / ..A\ 



rays of light showing the different degrees ' ^^^H^ vj^^""" -tlV 



of refraction they suffer in passing through ^^^igj^- ^^^r^ 



the humors of the eye (re n). ^^^mm^^ 



5699. The globe of the eye is composed of coats, chambers, and humors, and is operated on in its move- 

 ments by muscles. It may be considered as forming a large cup posteriorly, with a smaller cup applied 

 to its margin anteriorly ; or as though the segment of a large sphere were adapted to that of a smaller 

 one. The substance which gives figure and consistence to the larger segment is the sclerotic coat (l), 

 which is very firm and fibrous. The anterior cup or segment is supplied by the cornea, which is transpa- 

 rent, and formed of thin concentric plates of very different degrees of convexity in different animals, 

 and often of similar animals; to a defect in which is ascribed the indistinct vision or starting of some 

 horses. The cornea (a) is vascular and sensible, and in an inflamed state it admits the red blood, as we 

 see by the universal redness over the whole ; at other times it admits only the colorless parts of that 

 fluid. Immediately \vithin"the sclerotic coat is a thin vascular membrane, called the choroides (k), which 

 is spread over it nearly as far as the cornea, where it turns in and expands into the ciliary processes. It 

 also, by a peculiar fold, forms a ligament, after which it produces another projection into the cavity of 

 the eye, termed the uvea. It is here continuous, and presents a veil perforated in the centre. 



5700. The pupil of the eye (g) is the perforation which is seen annular in the human, oblong in the 

 horse, ox, and sheep, and perpendicular in the cat. The anterior surface of the uvea is covered with a 

 membrane, termed iris, on which the color of the eye depends ; in man it is grey, brown, black, 

 or blue; in the horse it is usually brown, but now and then white, when the animal is said to bewail, 

 eyed. At the central margin of the iris are seen, in a strong light, some little globular bodies or bags, 

 covered with a black pigment. They are usually attached to the upper margin only, but when any exist 

 on the lower they are small ; they have been mistaken for disease. The iris {e,f) is capable of accom- 

 modating itself to circumstances ; that is, it can enlarge the diameter of the central aperture or pupil {g) 

 so as to admit or shut out the rays of light. Over the central surface of the choroid expansion is spread 

 a dark mucous substance, called nigrum pigmentum. In animals, whose vision is distinct at night, this 

 pigment is found of a lighter color ; in man it is very dark, and his crepuscular vision is, therefore, 

 indistinct. In the grazing tribes it is of a greenish cast, lost in azure blue ; in the predaceous tribes it is 

 still lighter. Under this pigment is the mucous expansion, peculiar to quadrupeds, called tapetum. The 

 optic nerve (m) penetrates the sclerotic coat, and becomes expanded on its inner surface, in a membranous 

 lamen of exquisite fineness, called retina. On this it is supposed objects are painted, and thus taken 

 cognizance of by the brain. 



5701. The humors of the eye are the vitreous, the crystalline, and the aqueous. The vitreous humor 

 (hh) is of a jelly-like consistence, and occupies all the globe, except those parts taken up by the other 

 humors. The crystalline humor forms a lenticular body of moderate consistence, and is, therefore, 

 more pronerly called a lens (h). It is doubly convex (c, d), its posterior side resting in a concavity of the 

 vitreous numor. It is not of equal consistence throughout, being much firmer in the middle. Different 

 animals have the lens of different figures, to suit the purposes of their existence : in fishes it is nearly 

 spherical, but in quadrupeds, lenticular. It is a diseased opacity of this body that forms cataract The 

 aqueous humor is a limpid fluid which fills up the spaces not occupied by those already described. 



5702. The muscles of the eye. The motions of the eye ball are operated by seven muscles ; four recti or 

 straight, which elevate, depress, and draw to and from ; two oblique, which rotate the eye, and a retrac- 

 tor or choanoid, which is peculiar to quadrupeds, to draw the eye within the socket, and thus preserve it 

 from danger, draws the globe onwaids. 



5703. The phenomena of vision. If the diagram be examined, it will be evident that the eye of the 

 horse presents an optical instrument of exquisite workmanship and mechanism, admirably fitted to collect 

 the luminous rays from the various objects around, and to transmit them with truth to the brain. U the 



3 M 3 



