INJURIES AND DISEASES OF THE SKULL, &c. 135 



These muscles discharge another and a most important office. If we examine 

 near and distant objects through a telescope, we must alter the focus ; i. e., we 

 must increase or diminish the length of the tube. We must shorten it a little 

 when we examine distant objects, because the rays, coming to us from them in 

 a less divergent direction, are sooner brought to a point by the power of the 

 lens. Thus the straight and retractor muscles drawing back the eye, and forcing 

 it upon the substance behind, and in a slight degree flattening it, bring 

 the lens nearer to the retina, and adapt the eye to the observation of distant 

 objects. 



Still, however, being constantly employed in supporting the weight of the 

 eye, these muscles may not be able to turn it so rapidly and so extensively as 

 the wishes or wants of the animal require ; therefore two others are superadded 

 which are used solely in turning the eye. They are called oblique muscles, 

 because their course is obliquely across the eye. The upper one is most 

 curiously constructed, a, b. It comes from the back part of the orbit, and 

 takes a direction upwards and towards the inner side, and there, just under 

 the ridge of the orbit, it passes through a perfect mechanical pulley, and, turn- 

 ing round, proceeds across the eye, and is inserted rather beyond the middle of 

 the eye, towards the outer side. Thus the globe of the eye is evidently directed 

 inward and upward. Something more, however, is accomplished by this singu- 

 lar mechanism. The eye is naturally deep in the orbit, that it may be more 

 perfectly defended ; but it may be necessary occasionally to bring it forward, 

 and enlarge the field of vision. The eye is actually protruded under the 

 influence of fear. Not only are the lids opened more widely, but the eye 

 is brought more forward. How is this accomplished ? There are no muscles 

 anterior to, or before the eye — there is no place for their insertion. The object 

 is readily effected by this singular pulley, 6, c. By the power of this muscle, 

 — the trochlearis, or pulley -muscle — and the straight muscles at the same time 

 not opposing it, or only regulating the direction of the eye, it is really brought 

 somewhat forward. The lower oblique muscle rises just within the lacrymal 

 bone (i, p. 110), and, proceeding across the eye, is fixed into the part of the 

 sclerotica opposite to the other oblique muscle, and it turns the eye in a con- 

 trary direction, assisting, however, the upper oblique in bringing the eye forward 

 from its socket. 



CHAPTER VII. 



INJURIES AND DISEASES OF THE SKULL-THE BRAIN-THE 

 EARS— AND THE EYES. 



We have now arrived at a convenient resting-place in our somewhat dry but 

 necessary description of the structure of the horse, and we willingly turn to 

 more practical matter. We will consider the injuries and diseases of the parts 

 we have surveyed. In entering, however, on this division of our work, we 

 would premise, that it is impossible. for us to give the farmer such an account 

 of the nature and treatment of the diseases of horses as will enable him with 

 safety to practise for himself, except in the commonest cases. The causes ot 

 most diseases are so obscure, their symptoms so variable and their connexion 

 with other maladies so complicated and mysterious, that a life devoted to pro- 

 fessional study will alone qualify a man to become a judicious and successful 



