860 THE SENSE ORGANS AND SKIN OF THE HORSE 



junctiva are branches of the ophthalmic and facial arteries, and the blood is drained 

 awaj' by corresponding veins. The sensory nerves are branches of the ophthalmic 

 and maxillary divisions of the trigeminus. The motor nerves to- the orbicularis 

 oculi, corrugator supercilii, and malaris come from the facial nerve; the levator 

 palpebrse superioris is innervated by the oculomotor nerve and the unstriped 

 muscle of the lids by the sympathetic. 



THE LACRIMAL APPARATUS 



The lacrimal apparatus (Apparatus lacrimalis) comprises: (1) the lacrimal 

 gland, which secretes the clear lacrimal fluid; (2) the excretory ducts of the gland; 

 (3) the two lacrimal ducts or canaliculi, lacrimal sac, and naso-lacrimal duct, which 

 receive the fluid and convey it to the nostril. 



The lacrimal gland (Glandula lacrimalis) is situated between the supraorbital 

 process and the dorso-lateral surface of the eyeball (Fig. 563) . It is flattened, oval 

 in outline, and measures about two inches (ca. 5 cm.) transversely and an inch or 

 more (2.5-3 cm.) in the sagittal direction. Its superficial face is convex and is 

 related to the concave lower surface of the supraorbital process. The deep face is 

 concave in adaptation to the eyeball, from which it is separated by the periorbita. 

 The excretory ducts (Ductuli excretorii) are very small and are twelve to sixteen 

 in number; they open into the lateral part of the conjunctival sac along a line a 

 little in front of the fornix conjunctivae superior. In appearance and structure the 

 gland resembles the parotid. It receives its blood-supply chiefly from the lacrimal 

 artery. The sensory nerve is the lacrimal, and the secretory fibers are derived 

 from the sympathetic. 



The puncta lacrimalia are the entrances to the two lacrimal ducts. Each is a 

 fine, slit-like opening (about 2 mm. long), situated close behind the free edge of the 

 lid and about a third of an inch (ca. 8 mm.) from the medial canthus. The lacri- 

 mal ducts (Ductus lacrimales), upper and lower, begin at the puncta and converge 

 at the medial commissure to open into the lacrimal sac. The latter (Saccus lacri- 

 malis) may be regarded as the dilated origin of the naso-lacrimal duct. It occupies 

 the funnel-like origin of the bony lacrimal canal, which is termed the fossa of the 

 lacrimal sac, and leads to the naso-lacrimal duct (Ductus nasolacrimalis), which 

 passes forward and a little downward along the outer wall of the frontal sinus and 

 the nasal cavity and opens near the lower commissure of the nostril. Its length 

 is about ten to twelve inches (ca. 25-30 cm.). In the first part of its course it is 

 enclosed in the osseous lacrimal canal; further forward it lies in the lacrimal groove 

 of the maxilla, covered at first by a plate of cartilage and then by the mucous mem- 

 brane of the middle meatus (Fig. 59). The terminal part (Fig. 452) lies in the 

 ventral turbinate fold and opens on the skin of the floor of the nostril near the tran- 

 sition to mucous membrane. Accessory openings may occur a little further back. 



The first part of the duct, about 6 to 7 mm. in diameter, extends in a gentle curve, convex 

 dorsally, from the medial commissure toward a point just above the level of the infraorbital fora- 

 men. The second part (isthmus) is narrower (ca. 3-4 mm.); it extends forward and a little 

 ventrally about to a transverse plane through the first cheek tooth and lies in the groove above 

 the ventral turbinate crest. Beyond this the duct inclines upward and widens very consider- 

 ably, crosses the nasal process of the premaxilla obliquely, and contracts at its termination. The 

 mucous membrane may present valvular folds, the most distinct of which is situated at the 

 origin. 



THE PERIORBITA 



The periorbita^ is a conical fibrous membrane which encloses the eyeball with 

 its muscles, vessels, nerves, etc. Its apex is attached around the optic and orbital 

 foramina, and its base is in part attached to the bony rim of the orbit, in part con- 



1 Also known as the ocular sheath. 



