THE LACRIMAL APPARATUS 737 



cartilage has an elongated (juadrilateral outline. The part of it which lies in the 



membrana is wide and thin. The deep part is narrower and thicker and is 



embedded in fat at the inner side of the eyeball. Numerous minute lymph nodules 



occur in the membrana nictitans, and the deep part of the cartilage is surrounded 



by a gland which reseml)les the lacrimal gland in structure (Cdandula superficialis 



palpebrse tertiae). 



Ordinarily the tliird eyelid extemls verj- little over the inner end of the cornea, but when the 

 eyeball is strongly retracted (.as in tetanus) the membrana is protruded over it so as to measure 

 about an inch (ca. 2 to 3 cm.) in its middle. This effect results from the pressure of the ej'eball 

 and its muscles on the fat which surrounds the deep part of the cartilage. 



Vessels and Nerves. — The arteries which chief!}- supply the eyelids and con- 

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

 away by corres]ionding veins. The sensory nerves are branches of the ophthalmic 

 and maxillary divisions of the trigeminus. The motor nerves to the or])i('ularis 

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

 palpebrae superioris is innervated by the oculomotor, 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. 438). It is flattened, oval 

 in outline, and measures al:)0ut two inches (ca. 5 cm.) transversely and an inch or 

 more (2.5 to 3 cm.) in the sagittal direction. Its upper face is convex and is related 

 to the concave lower surface of the stipraorbital process. The lower face is concave 

 in adaptation to the eyeball, from which it is separated l^y the periorbita. The 

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

 munl)er; they open into the outer 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 chieflj- 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 internal canthus. The lacri- 

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

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

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

 the funnel-like origin of the bony lacrimal canal, and leads to the naso-lacrimal duct 

 (Ductus naso-lacrimalis), which passes forward and a little downward along the 

 outer wall of the frontal sinus and the nasal cavity and opens near the lower com- 

 missure of the nostril. Its length is about ten to twelve inches (ca. 25 to 30 cm.). 

 In the first part of its course it is inclosed 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 membrane of the middle meatus. The terminal 

 part lies in the inferior tm*1)inal fold and opens on the skin of the floor of the nostril 

 near the transition to nmcous membrane. Accessory openings may occur a little 

 further l)ack. 



The first part of the duct, al)out G to 7 mm. in diameter, extends in a gentle curve, convex 

 dorsally, from the internal commissure toward a point just above the level of the infraorbital 

 foramen. The second part (isthmus) is narrower (ca. 3 to 4 mm.); it extends forward and a 

 47 



