862 



THE SENSE ORGANS AND SKIN OF THE HORSE 



larlj- the medial and lateral 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 dorsal oblique raises the lateral 

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

 draws the eyeball backward, and its parts may separately reinforce the correspond- 

 ing recti. Also the four recti acting together ^^411 retract the eyeball. 



The actual movements of the eyeball are by no means so simple as might be inferred from 

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

 actions of several muscles, in^'olving combinations which are quite complex and difficult to analyze 

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



Fig. 692. — Vertical Axial Section of Orbit or Horse. 

 a. Eyelids; 6, fascia bulbi; c, c', retractor oculi; d, rectus ventralis; e, obliquus ventralis (in cross-section); /, 

 rectus dorsalis; g, levator palpebrae superioris; h, obliquus dorsalis (in cross-section); i, lacrimal gland; k, k' , peri- 

 orbita; I, superficial fascia; m, deep fascia; n, skin; o, retrobulbar fat; p, extraorbital fat; q, temporalis muscle; r, 

 supraorbital process; s, cranial wall; 1, cornea; 3, sclera; 5, chorioidea; 4, ciliary muscle; 5, iris; 6, granula iridis; 

 7, retina; 7', optic papilla; 5, optic nerve; 5, crystalline lens; .(0, capsule of lens; i I , ciliary zone ; i^, posterior cham- 

 ber; i 5, anterior chamber; i 4, conjunctiva bulbi; i 5, vitreous body. (After EUenberger, in Leisering's Atlas.) 



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 lateralis and obliquus dorsalis, 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. 

 It is protected in front by the eyelids and conjunctiva, and in its middle by the 

 complete orbital ring, and is related behind to the fascia bulbi, fat, and ocular 

 muscles. 



It has the form approximately of an oblate spheroid, but is composed of the 

 segments of two spheres of different sizes. The anterior transparent segment, 

 which is formed by the cornea, has a radius of curvature of about 17 mm., and the 



