A Treatise on the Eye/Book 1/Chapter 3
CHAP. III.
Of the Glandula Lachrymalis.
Sect.
Pag.
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| 1. | The Lachrymal Gland, what, | 39 |
| 2. | Its Situation and Excretories, | 39 |
| 3. | Its Use is to separate the Tears for washing and cleaning the Cornea, | 40 |
| 4. | This Liquor upon particular Occasions is squeezed out in greater plenty by the Contraction of the orbicular Muscle of the Eye-lids, and why, | 41 |
CHAP. III.
Of the Glandula lachrymalis.
Sect. 1. BY the lachrymal Gland, I do not here mean that Caruncle or Carnous Tubercle, situate in the great Angle of the Eye, at the Entry into the lachrymal Sac, which by the Antients was called the Glandula lachrymalis, and whose Use in Man is not so much to furnish the lachrymal Juice for moistening and cleaning the Eye, as to keep the Puncta lachrymalia, betwixt which it is placed, open, when the Eyes are shut, and to hinder the Tears from running down upon the Cheeks at the great Angle of the Eye: But, by the Glandula lachrymalis, I underdtand that Gland commonly called the Glandula innominata, which is situate behind the Conjunctive, upon the upper Part of the Globe, in a hollow Sinus formed in the Orbit itself, for the more convenient lodging of this Gland.
§ 2. This Gland is very large, and extends from the external Angle of the Eye to near its Middle; but it is not always exactly of the same Figure.
It is of the conglobate Sort, being made up of many simple Glands, which are so united together, as to form as it were several Lobes, sometimes more, sometimes fewer: From each Lobe there arises a small Excretory, which opens upon the internal Side of the upper Eye-lid. These Excretories were first discovered in Sheep and Calves, by that accurate Anatomist Nicolaus Steno, who also points out the Manner how they are to be found. (Obfervat. de Gland. Oculor.) In the Eye of an Ox, I have frequently seen eight or nine such Excretories, each of which was large enough to admit of a Hog's Bristle: But in Man these Excretories are so small, that their Cavity can scarce be observed, and are like so many small Lines going from the Gland to the internal Side of the Eye-lid where they terminate.
§ 3. Some Anatomists, Morgagni himself not excepted, have been so scrupulous, as to doubt whether these Lines are the Excrctories of the Glandula lachrymalis, because of their smallness, and the invisibility of their Cavities. But with how little Reason, appears from the Analogy betwixt these Lines, and the Excretories of this Gland in Brutes; more especially, that there is no other visible Way how the Liquor secerned in this Gland can be conveyed from it: We therefore conclude, that the Use of this Gland is to separate from the Blood that thin, transparent, saltish Liquor called Tears, and to send it by these line-like Excretories, which open upon the Inside of the superior Eye-lid, to the anterior Part of the Eye, for moistening, washing and cleaning it, which otherwise would soon dry, wrinkle and lose its Transparency, by the constant Action of the Air, and the Dust, Smoke and Moats contained in it.
§ 4. This Liquor is continually, tho' but in small Quantity, poured out upon the Eye, and, by the twinkling Motion of the Eye-lids, is equaly diffused over the whole Cornea, for keeping it moist, clean and transparent; but, upon particular Occasions, when our Eyes are any way darkened, or suffer any Pain or Itching, by being long exposed to drying Winds, Smoak or Dust, this Liquor is poured out upon the Eyes in greater Plenty; because then, as has been before observed, the orbicular Muscle contracts with a more than ordinary Force, and, by pressing the Globe of the Eye inward, compresses the lachrymal Gland, and expresses the Liquor therein contained; and, when the Action of this Muscle is not sufficient for expressing a sufficient Quantity of this Liquor, then we rub our Eyes with. our Hands, which presses the Eye yet more inwards upon this Gland, and causes it yield its Liquor in greater Plenty. Whence we may see, why all Animals, whose Eyes are exposed to the Injuries of the Air, and who want Hands for rubbing their Eyes, are provided with an interior Eye-lid, to which at the great Canthus is attached a certain Gland of the lachrymal Kind, which is peculiar to such Creatures, and which opens by one or more Excretories upon the Eye, about the Middle of this Eye-lid; for this Eye-lid, by moving transversly to and again upon the Eye, while the other Eye-lids shut and open, stretches and agitates this Gland, and expresses the lachrymal Juice contained in it, for washing and cleaning the Cornea as Occasion requires, and is a sufficient Provision for the Want of Hands, wherewith Man and the Ape rub there Eyes upon the like Occasion, as has been before observed.