INJURIES AND DISEASES OF THE SKULL, ETC. 167 



The power of tliis muscle is very great. It renders some operations on 

 the eye almost impossible. It is an admirable substitute for the want of 

 hands, to defend the eye from many things that would injure it ; and, 

 being partially separated into four divisions, it assists the straight muscles 

 in turning the eye. 



These muscles discharge another and a most important ofRce. If we 

 examine near and distant objects through a telescope, we must alter the 

 focus ; i. 6. 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 powder of the lens. Thus the straight and retractor muscles draw- 

 ing 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 exten- 

 sively 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, h. It comes from the back 

 pai^t 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 turning round, proceeds across the eye, under the 

 tendon of the upper straight muscle, and is inserted rather beyond the 

 middle of the eye, towards the outer side. Thus the globe of the eye is 

 evidently directed dowTiwards and outwards. Something more, however, 

 is accomplished by this singular mechanism. The eye is naturally deep in 

 its orbit, that it may be more perfectly defended ; but it may be necessary 

 occasionally to bring it foi-ward, 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 eff'ected by this sin- 

 gular 23ulley, 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 lachrymal bone (i, p. 166), 

 and, proceeding across the eye, is fixed into the part of the sclerotica op- 

 posite to the other oblique muscle, and it turn." the eye in a contrary 

 direction, assisting, however, the upper oblique in bi inging the eye forward 

 from its socket. 



CHAPTER IX 



INJURIES AND DISEASES OF THE SKULL — THE BKAIN — 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 vvdllingly 

 turn to more practical matter. We -will consider the injuries and diseases 

 of the parts we have surveyed. In entering, however, on this di-vision of 

 our work Ave 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 mth safety to practise for himself, except in the com- 



