THE EYEBALL 



739 



much sliorter than the recti. It arises from the inner wall of the orbit in the small 

 depression (Fossa muscularis) behind the lacrimal fossa. It curves around the rectus 

 inferior and is inserted into the sclera near and partly beneath the rectus externus. 

 Actions. — The superior and inferior recti rotate the eyeball about a transverse 

 axis, moving the vertex of the cornea upward and downward respectively. Simi- 

 larly the internal and external recti rotate the eyeball about a vertical axis, turning 

 the vertex of the cornea inward and outward respectively. The oblique muscles 

 rotate the eyeball about a longitudinal axis; the superior oblique raises the outer 

 end of the pupil, while the inferior oblique lowers it. The retractor as a whole 

 draws the eyeball l)ackwar(l. and its parts may se])arately reinforce the correspond- 

 ing recti. Also the four recti acting together will retract the eyeball. 



Fig. 555. — Vkrtical Axial Skctio.n' of Orhit of Horse. 

 a. Eyelids; b, bulbar fascia (Tenon's capsule); c, c' , retractor bulhi; (/, rectus oculi inferior; c, obliquus 

 oculi inferior (in cross-section); /, rectus oculi superior; g, levator palpebrap superioris; h, obliquus oculi superior 

 (in cro.ss-.section) ; >', lacrimal gland; k, k' , periorbita; /.superficial fascia; m.deep fa.scia; n, skin; o, retrobulbar 

 fat; p, extraorbital fat; q, temporalis muscle; r, supraorbital process; s, cranial wall; /.cornea; 2, sclera; 

 3, chorioidea; 4, ciliary muscle; 5, iris; 6, granula iridis; 7, retina; 7', optic ))apilla; 8, optic nerve; 9, crys- 

 talline lens; 10, capsule of lens; 11, ciliary zone; 12, posterior chamber; l.i anterior chamber; 14, conjunctiva 

 bulbi; Jo, vitreous body. (After EUenberger, in Leisering's Atlas.) 



The actual movements of the eyeliall are by no means so simple as might be iiiferrefl from 

 the foregoing general statements. Practically all movements are produced by the coordinated 

 actions of several muscles, involving combinations which are quite complex and difficult to analyze 

 accurately. Further complication is caused by the fact that the recti are not inserted at equal 

 distances from the equator and the axes of rotation of the oblique muscles do not correspond to 

 the longitudinal axis of the eyeball. 



Nerve-supply. — The oculomotor nerve supplies the foregoing muscles, with 

 the exception of the rectus externus and obliquus superior, which are innervated by 

 the abducens and trochlearis respectively. 



THE EYEBALL 

 The eyeball (Bulbus oculi) is situated in the anterior part of the orbital cavity, 

 protected in front by the eyelids and conjunctiva, in its middle by the complete 

 orbital ring, and related behind to the bulbar fascia, fat, and ocular muscles. 



