CHAP. XVII.] THE EYELtDS. 4 1 



wards, under the inferior rectus, is inserted into the sclerotic, near, 

 but beneath the superior oblique. The action of the recti muscles 

 is obvious: when used in concert, they fix the eyeball; when 

 singly, they turn it towards their respective sides. The globe, besides 

 being imbedded in fat, is suspended or slung in a capsule of fibrous 

 tissue, with which it is in immediate contact. This is attached in 

 front to the tarsal cartilages, and is prolonged backwards over the 

 globe and optic nerve, after being perforated by the muscles. Mr. 

 O'Ferrall, who has lately directed attention to this fibrous struc- 

 ture, has termed it the tunica vaginalis oculi. It is important, 

 as furnishing support to the eyeball under muscular movements. 

 The recti muscles are supplied by the third pair of nerves, ex- 

 cept the external, which receives the sixth. The oblique muscles 

 antagonize the recti, and must in addition, if acting together, 

 draw the globe inwards, and converge the axes of the eyes. The 

 superior oblique, if alone, would most probably direct the front 

 of the eye downwards and outwards, and the inferior oblique up- 

 wards and inwards; but on these points much difference of opinion 

 still prevails. The former is supplied by the fourth pair, the latter 

 by the branch of the third that gives the motor fibres to the ophthal- 

 mic or lenticular ganglion, from which the ciliary muscle and iris 

 receive their nerves. And, in connexion with the latter arrangement, 

 it is interesting to remark that the pupil contracts when the eyes are 

 directed inwards and upwards, and generally also in the adjustment 

 for near vision, which is attended with a convergence of the optic 

 axes. During sleep the eyes are usually turned inwards and upwards, 

 and the pupil is contracted actions produced through the medium 

 of the inferior division of the third pair supplying the inferior oblique 

 and iris. The iris is evidently involuntary in its movements ; it 

 contracts only in obedience to the stimulus of light upon the retina, 

 or when the eye is turned upwards and inwards. 



The eyelids are exquisitely adapted to shield the eye from too 

 strong light, and to protect its anterior surface from the contact of 

 hurtful substances. In the upper lid, which is much larger and 

 more moveable than the lower, there is a thin sheet of cartilage, 

 curved to fit the front of the eye, and to facilitate its gliding motion 

 over the globe. To the. posterior convex border of this tarsal car- 

 tilage the levator palpebrse muscle is attached, which thus serves to 

 elevate the whole lid. The lower lid possesses a very narrow slip 

 of cartilage, which meets the upper at each side through the medium 

 of fibrous tissue, which, at the outer angle of the eye is attached 

 loosely to the malar bone, and at the inner angle forms a tendinous 



