THJB ETS.] 



INIODERN VETERINARY PR ACT I TK. 



[tiik kyk. 



CnAPTER XV. 



TUE Eii: AND ITS DISEASES. 



THE EYE IN GENERALr— THE MUSCLES OF 

 THE EYE. 



Ik speakinc: of tlio muscles of tho eye, tlie 

 orbicularis first comes under our notice. This 

 muscle surrounds, and is attached to the bones 

 of the orbit. It passes under the loose skin 

 of the upper eyelid, which has great motion ; 

 and its ollice is to close the eyelid perfectly, 

 or to assist in ])erforming the involuntary act 

 of winking. The antagonistic muscle to tliis 

 is the levator paljjelrce, rising from the pos- 

 terior part of the orbit, and passing over the 

 orbit, contiguous to the lachrymal glands, 

 which are placed in the hollow of the orbit 

 above the eye. This pair of muscles is in 

 almost unceasing action, and like all other 

 muscles would become weary ; but by acting 

 in opposition they obtain intervals of rest. 

 Thus, when the eyes are shut, the orbicularis 

 is in action ; but when awake, the levator 2^(il- 

 pebrcs. This last, however, is relaxed by the 

 act of winking, which is clearly one intention 

 of winking: another is to allow the tears to 

 pass over the orbit for the pur;iose of removing 

 extraneous matter, by means of the action of 

 the eyelids. 



To accomplish the various movements of the 

 eye, there are four straight muscles, two 

 oblique, and one retractor, or drawing-back 

 muscle, the powers of which are very supe- 

 rior to those in the muscles of the human 

 subject. Returning, however, to the straight 

 muscles, we commence with the levator ones, 

 which rise from the posterior part of the 

 orbit, pass over the retractor muscle, and are 

 attaclicd to the sclerotic coat. They are for the 

 purpose of directing the eye upwards. Tiie 

 eye always acts in unison with these muscles. 

 The next are the abductor, which originate in 

 the posterior part of the orbit, and are inserted 

 into the anterior part of the sclerotic coat 

 on the outside. These direct the eye from 

 the nose. The depressors rise in the pos- 

 terior part of the orbit, and are inserted into 

 the inferior part of the sclerotic coat. The 



adductors have the same origin, and are inserted 

 into the sclerotic coat towards tho inner can- 

 thus. All these muscles acting together, have 

 the power of drawing the eye into the orbit; 

 but their action is inconsiderable. 



Tho oblique muscles are two. One takes its 

 rise from the inferior part of the orbit, and ia 

 inserted into the sclerotic coat below. Tho 

 other, called the irochliaris, also rises in tho 

 same part, and going through a pulley-like 

 adaptation, is inserted into the anterior part 

 of the sclerotic coat, and thus draws the eye 

 forward. The principal use of these muscles 

 is to act in opposition to those which are 

 straight, by drawing the eye forward. 



The retractor occult, rising from the supe- 

 rior part of the orbit, surround the optic nn-ve, 

 but are separated from it by adeps (i. e., lut) ; 

 and that the fibres may not aflect thu nerve, 

 they are not attached to the sclerotic coat, 

 where the nerve emerges. Here we may men- 

 tion how the inner and outer canthus may be 

 distinguished. AVhen the eyes are separated 

 from the body, it is the broad side which forms 

 the inner canthus, over which the haw plays. 

 In the structure of birds, for example, there is 

 an organisation somewhat similar to tho haw 

 in the horse, called the membrana nictitans, a 

 semi-transparent membrane, having a muscle 

 and a tendon. In the horse this is carlilnir'^ 

 edged with black, at the anterior part fasteiK i 

 to the conjunctiva by the cellular membrane ; 

 and if an attempt is made to pass the finger 

 between the eye and the haw, the action will 

 be prevented by the conjunctiva, showing that 

 the conjunctiva is a reflected membrane over 

 the cellular attachment. Tho haw is divided 

 into two parts. The anterior part is convex 

 on the outer surface, and concave on the inner, 

 to fit the convexity of the eye. The haw acts 

 as a kind of shield ; but not being suflicieut to 

 cover tlie whole orb, whilst in its natural situa- 

 tion, the retractor muscle acts first, by whicli 

 means the ball of the eye presses upon tho 

 adeps, and the haw, in consequence, is forced 



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