970 



PRACTICE OF AGRICULTURE. 



Part III. 



6368. The form of the ear is dependent on the concha cartilage, which is found pointed and small in the 

 Arabian, but large and broad in the heavy breeds. The cavity within the concha is thrown into folds 

 throughout, which increases its surface, and reflects the sonorous waves. This outer ear is attached to 

 the internal, by connecting cartilaginous portions and appropriate ligaments. The parts of the internal 

 ear are, the meatus auditorius intdrnus, or pa.ssage ; the membrtlna tympani, or separating membrane 

 between the external and internal parts : the tympanum, drum, or barrel of the ear ; and the labyrinth. 

 The Eustachian, tube is an opening at the upper and anterior edge of the hollow of the tympanum, form- 

 ing a duct which is in part bony, and in part cartilaginous; extending from the tympanum to a large and. 

 peculiar cavity at the posterior part of the nasal fossa. 



6369. The sense of hearinfi, is formed through the medium of the expansion of the soft portion of the. 

 auditory nerve over the internal ear ; sounds, therefore, entering the cavity of the concha, are reflected 

 alternately from its sides into the tympanum, whose oscillations are imparted to the brain. 



SuBSECT. 8. The Eye and its Appendages. 



6370. The appendages to the eye are, first, a funnel-shaped cavity formed by the concurrence of the bones 

 of the skull, called the orbit, not placed directly in front as in man, but inclining laterally, to enable the' 

 animal to onbrace a larger field of view. The eyelids are an upper and under, of which the upper is the 

 most considerable, and 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 muscles. Attached 

 to the edge of each is a cartilaginous rim, called the tarsus. The cilia, or eyelashes, are not, as in man, 

 above and below ; the upper lid only is furnished with hairs, and these are not placed in one row, 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 ; it is fur- 

 nished with five or six excretory ducts, which secrete the lachrymae or tears to lubricate the surface of 

 the globe. The superfluous tears are carried off by two openings at the inner angle, called pUncta la- 

 chrym,hlia, by which means the tears are at once carried into the nose, and not as in man first into a 

 lachrymal sac. The carihicula lachrymalis 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 hair, or nictating membrane, is an im- 

 portant 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 inner 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-evi), by the action of the retractor muscle ; and under inflam- 

 mation 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 nictitating membrane cannot be for a moment 

 dubious. It is denied to man and to monkeys, because they, having hands, can with their fingers remove 

 dust and dirt from the eyes ; but to the horse and most other quadrupeds it is essentially necessary for 

 these purposes. 



832 



6371. A diagrayn of the eye {fig. 8.32.) dis- 

 plays the transparent cornea in front of 

 its globe {a), tlie crysUlline lens (6), its 

 posterior convexity (c), its anterior con- 

 vexity (rf), the iris, or curtain {e,f), the 

 anterior chamber occupied by the aqueous 

 humour of the pupil {g), the posterior 

 chamber filled with the vitreous humour 

 [h h), the retina (i), the choroid coat (Ar), 

 the sclerotic coat (Z), and optic nerve (w^), 

 rays of light showing the different degrees 

 of refraction they suffer in passing through 



771^ the humours of the eye (w). 



6372. The globe of the eye is composed of 

 coats, chambers, and humours, and is ope- 

 rated on in its movements 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 scle- 

 rotic coat (/), which is very firm and fibrous. The anterior cup or segment is supplied by the cornea, 

 which is transparent, and formed of thin concentric plates of verv different degrees of convexity 

 in different animals, and often in similar animals; to a defect in which is ascribed the indistinct vision 

 or starting of some horses. The cornea () 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 

 colourless parts of that fluid. Immediately within the sclerotic coat is a thin vascular membrane, called 

 the choroidcs (k), which is spread over it nearly as far as the C('>rnea, where it turns in and expands into 

 the ciliary processes. It also by a peculiar fold forms a ligament, after which it produces another projec- 

 tion into the cavity of the eye, termed the uvea. It is here continuous, and presents a veil perforated in 

 the centre. 



6373. The pupil of the eye {g) is the perforation which is seen annular in the human, oblong iti 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 colour 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 be wall- 

 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 colour : in man it is very dark, and his crepuscular vision is, therefore, in- 

 distinct. 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 tapctutn. 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. 



6374. The humours of the eye are the vitreous, the crystalline, and the aqueous. The vitreous humour 

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

 humours. The crystalline hu?nour forms a lenticular body of moderate consistence, and is, therefore, more 

 properly called a lens (6). It is doubly convex (c, d), its posterior side resting in a concavity of the vitreous 

 humour. 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 

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



6375. The muscles of the eye. The motions of the eyeball 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 retractor 



