3. Lachrymal Apparatus.—The lachrymal gland is a large reddish gland, not lobulated externally, which lies on the surface of the eyeball just beneath the lateral angle of the eye. The gland is situated immediately craniad of, and in contact with, the zygomatic process of the frontal bone ([Fig. 154], 11, [page 374]). It is flat and about one centimeter in diameter. The ducts which pass from the gland to the eye are not visible to the naked eye. The fluid secreted by the gland collects at the medial angle of the eye and passes into the two openings of the lachrymal canals, one of which is found on the pigmented edge of each lid, three or four millimeters from the medial angle of the eye. The canals passing from these openings soon unite to form the nasolachrymal duct, which passes through the lachrymal bone, along the medial surface of the maxillary, to open into the nasal cavity ventrad of the ventral concha of the nose.

4. Muscles of the Orbit ([Fig. 166]).—Within the orbit are eleven muscles, ten connected with the eyeball, and one, M. levator palpebræ superioris, with the upper eyelid.

Of the ten muscles of the eyeball, eight are straight muscles, passing from an origin about the inner end of the orbit directly distad; the other two have an oblique direction. Four of the straight muscles are larger and are known as the Mm. recti (b, c, d); these are distinguished according to their position as lateral (c), medial, superior (d), and inferior (b). The four smaller straight muscles constitute together M. retractor oculi (e). The two oblique muscles are known as Mm. obliqui and are distinguished according to their position as inferior (a) and superior.

[Fig. 166].—Muscles of the Eyeball, Lateral View.

a, M. obliquus inferior; b, M. rectus inferior; c, M. rectus lateralis; d, M. rectus superior; e, parts of M. retractor oculi; f, the oculomotor nerve.

Mm. recti (b, c, d).—The four recti muscles arise from the bone about the optic foramen and pass toward the eyeball. They are inserted by thin, flat tendons along a line which separates the darker caudal part of the sclerotic from the white zone of the sclerotic which borders the cornea,—the line of insertion of the four tendons forming thus a circle about the eyeball. The rectus superior (d) is partly covered by M. levator palpebræ superioris. The tendon of the rectus inferior (b) is crossed by M. obliquus inferior (a). The recti muscles are all innervated by the oculomotor (third) nerve (f), except the lateral (c), which is supplied by the abducens (sixth) nerve.

M. retractor oculi (e).—The retractor oculi arises about the optic foramen and divides into four heads which lie nearer the eyeball than the recti and are therefore partly covered by the latter. They alternate with the recti and are inserted into the eyeball at about its equator, except the inferior division, which is inserted on a line with the recti. They are supplied by the oculomotor nerve.

M. obliquus inferior (a).—The inferior oblique arises from the maxillary bone just laterad of the lachrymal bone and curves over the ventral side of the eyeball along the lines of insertion of the recti, crossing the tendon of the rectus inferior (b) to the ventral edge of the tendon of the lateral rectus (c), where it is inserted. It rotates the eyeball and is supplied by the third nerve (f).

M. obliquus superior.—The superior oblique takes origin from the cranial border of the optic foramen, passes dorsocraniad, and ends in a small rounded tendon which passes through a fibrous pulley-ring. This ring is situated near the rim of the orbit, one or two centimeters craniad of the zygomatic process of the frontal bone, and is attached to the wall of the orbit by two fibrous bands. After passing through the pulley-ring the tendon turns and passes laterocaudad; it becomes much expanded, passes caudad of the superior rectus, and is inserted into the eyeball along the caudal margin of the insertion of the superior rectus tendon. The superior oblique is supplied by the trochlear (fourth) nerve.