The parts which constitute the lachrymal apparatus are the following, viz.: - The gland by which the tears are secreted, situated at the upper and outer side of the orbit, together with its excretory ducts; the two canals into which the fluid is received near the inner angle: and the sac with the nasal duct continued from it, through which the tears pass into the inferior meatus of the nose.
The lachrymal gland, an oblong flattened body, about the size of a small almond, is placed in the upper and outer part of the orbit, a little behind the anterior margin. The upper surface of the gland, convex, is lodged in a slight depression in the orbital plate of the frontal bone, to the periosteum of which it adheres by fibrous bands; the lower surface is adapted to the convexity of the eyeball, and is in contact with the upper and the outer recti muscles. The fore part of the gland, separated from the rest by a thin layer of fascia, and sometimes described, as a distinct gland (glandula lachrymalis minor, Rosenmüller), is closely adherent to the back of the upper eyelid, and is covered on the ocular surface merely by the conjunctiva; its lobules are small and separate, with minute ducts, some opening separately, others joining the ducts from the principal gland, which are also very small. The number from both divisions of the gland seldom exceeds twelve or fourteen. After running obliquely under the mucous membrane, and separating at the same time from each other, they open in a row at the fornix conjunctivae, by separate orifices, at its upper outer part. In minute structure the lachrymal gland resembles the salivary glands.
Lachrymal canals of the lacrymal apartus
On the margin of each lid, near the inner angle, and in front of the fold of membrane called plica semilunaris, is a small elevation (papilla lachrymalis), as already mentioned. Each papilla is perforated by a minute aperture, punctum lachrymale; and at these apertures commence two narrow canals, canaliculi, which convey the tears from the eye to the lachrymal sac. The upper canal is rather the smaller and longer of the two: it first ascends from the punctum; then makes a sudden bend, and is directed inwards and downwards to join the lachrymal sac. The lower canal descends from the corresponding punctum, and then takes a. nearly horizontal course inwards. Both canals are dilated where bent. In some cases they unite near the end; more commonly they open separately, but close together, into the sac.
The lachrymal sac and nasal duct constitute together the passage by which the tears are conveyed from the lachrymal canals to the cavity of the nose. The lachrymal sac (fig. 398, 2), the slightly dilated upper portion of the passage, is situated at the side of the nose, near the inner canthus of the eye, and lies embedded in a deep groove in the ungual and upper maxillary bones. Its upper end is closed and rounded, and the lower end gradually narrows into the nasal duct. On the outer side, and a little in front, it receives the lachrymal canals; and here it is placed behind the tendo palpebrarum, and some of the inner fibers of the orbicular muscle of the lids; while on its orbital surface is the tensor tarsi muscle. The sac is composed of fibrous and elastic tissues, adhering closely to the bones above mentioned and strengthened by fibrous processes sent from the tendo palpebrarum, which crosses a little above its middle. The inner surface is lined by a reddish mucous membrane, which is continuous through the canaliculi with the conjunctiva, and through the nasal duct with the raucous membrane of the nose.
The nasal duct (ductus ad nasum), about six or seven lines in length, grooving the upper maxillary bone, descends to the fore part of the lower meatus of the nose, the osseous canal being completed by the ungual and lower turbinated bones. A tube of fibrous membrane, continuous with the lachrymal sac, adheres to the parietes of this canal, and is lined by mucous membrane, which, at the opening into the nose, is often arranged 'so as to form an imperfect valve. The nasal duct is rather narrower in the middle than at either end; its direction is not quite vertical, but inclined slightly outwards and backwards.
The mucous membrane in the canaliculi possesses a stratified, scaly epithelium, but in the nasal sac and duct a ciliated epithelium as in the nose.
Various valves have been described in connection with the lachrymal sac and canals. One, the valve of Hasner, is formed by the mucous membrane of the nose overhanging the inferior orifice of the nasal duct, and has had imputed to it the function of preventing entrance of foreign matters in violent expiratory- movements ; but the disposition of the mucous membrane at this orifice appears to be subject to considerable variation. Another fold, the valve of Huschke, placed at the opening of the canaliculi into the lachrymal sac, is supposed by some to prevent the return of the tears from the sac into those tubes, but, by others, it is declared to be inconstant, and insufficient, even when present, to close the orifice. A third fold, the valve of Foltz, is described as forming a projection inwards on one side of the vertical part of each canaliculus, near the punctum lachrymale, and as being sufficient to close the tube when it is flattened by the pressure of the fibers of the orbicularis and tensor tarsi muscles as in winking. The experiments of Foltz on rabbits go to prove that the punctum lachrymale having been turned backwards towards the eye in winking, and the canaliculus being compressed by the muscles, as soon as the pressure is removed the canaliculus resumes its open form, and so sucks in tears which by the next compression in winking are forced onwards into the lachrymal sac : and also, that when the muscles are paralyzed, the canaliculi cease to carry away the tears. See review of Foltz's paper in Dublin Quarterly Jomnal, Feby. 1863 ; also, Hyrtl, Topogr. Anatomie.