862 THE SENSE ORGANS AND SKIN OF THE HORSE 
larly 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 will 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 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 
Fic. 692.—VerTICAL AXIAL SECTION OF ORBIT OF Horsr. 
a, Eyelids; b, fascia bulbi; c, ¢’, retractor oculi; d, rectus ventralis; e, obliquus ventralis (in cross-section); f, 
rectus dorsalis; g, levator palpebre superioris; h, obliquus dorsalis (in cross-section); 7%, lacrimal gland; /, k’, peri- 
orbita; /, superficial fascia; m, deep fascia; n, skin; 0, retrobulbar fat; p, extraorbital fat; gq, temporalis muscle; r, 
sclera; 3, chorioidea; 4, ciliary muscle; 4, iris; 6, granula iridis; 
alline lens; 10, capsule of lens; 11, ciliary zone; 12, posterior cham- 
supraorbital process; s, cranial wall; 1, cornea; 
7, retina; 7’, optic papilla; 8, optic nerve; 9, cry: 
ber; 13, anterior chamber; 14, conjunctiva bulbi; 15, vitreous body. (After Ellenberger, 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 
