104 



HEALTH AND DISEASE 



canthus of the eye, and is attached to a cushion of fat by its posterior 

 angle, while in front it presents a very thin edge by which small particles 



of dust, hairs, insects, and the like, 

 are removed. If not swept away by 

 the nictitating membrane, they are 

 washed away by the copious secretion 

 of tears which they excite from the 

 lachrymal gland (fig. 246, E). 



It is moved or rotated by seven 

 muscles, viz., the four recti, or 

 straight muscles, named respectively 



Fig. 239. Muscles of the Eyeball 



A, Superior Rectus Muscle. B, Retractor Muscle. ,1 cmnprinr infprinr p-jrtprrvil inrl 

 c, External Rectus Muscle. D, Inferior Rectus Muscle. Or > ] Or > ( tll > < 



E, Superior Oblique Muscle. F, Point of Insertion of 

 Inferior Oblique Muscle, o, Optic Nerve. H, Globe of 

 the Eye. i, Iris. J, Pupil. 



internal rectus; the superior and 

 inferior oblique muscles, and the 

 retractor bulbi. The superior oblique 



muscle, which turns the eye downwards and outwards, is supplied by the 

 fourth nerve or trochlearis. The external rectus is supplied by the sixth 

 cranial nerve, and the remaining five muscles are supplied by the third 



nerve. These muscles, with the 

 exception of the inferior oblique, 

 arise from the back of the orbit, 

 and, passing forwards, are inserted 

 into the fore part of the globe. 

 The inferior oblique muscle arises 

 from the inner part of the floor of 

 the orbit, and, running outwards 

 and upwards, forms a kind of sling 

 - supporting the eye together with 

 the tendon of the superior oblique. 

 The superior oblique (fig. 239, E), 

 arising from the back of the orbit, 

 ends in a tendon which runs through 

 a small loop or pulley on the upper 

 part of the inner wall of the orbit, 

 and then, changing its direction, 

 runs outwards and a little back- 

 wards, to be inserted into the upper 

 part of the globe. 



The ancient division of the structures of the eye into three coats or 

 tunics, and three humours, is still the most convenient for description. 

 The first coat is composed of the sclerotic and cornea (fig. 240, D, L). 



Fig. 240. Section of Eye 



A, Lachrymal Gland. B, Levator Palpebrae Superioris. 

 C, Levator Oculi. D, Sclerotic Coat. E, Choroid Coat. 

 F, Retina, o, Optic Nerve. H, Vitreous Humour. 

 I, Capsule of the Lens. J, Crystalline Lens. K, Aqueous 

 Humour. L, Cornea. M, Iris. N, Upper Eyelid. 



