ACTION OF THE MUSCLES OF THE EYEBALL. 251 



The great use of the tears would seem to be to moisten the conjunc- 

 tiva, and to remove extraneous bodies from its surface, thus assisting 

 the motions of the eyelids and .eyeball. The tears are secreted by the 

 lachrymal gland ; and, by means of its excretory ducts, are poured upon 

 the surface of the tunica conjunctiva, at the upper and outer part of 

 the eyeball. Their farther course towards the puncta lachrymalia has 

 been the subject of difference of sentiment. Many physiologists have 

 considered that, owing to the form of the tarsal cartilages, a canal 

 exists, when the eyelids are closed, of a triangular shape, formed an- 

 teriorly by the junction of the cartilages, and behind by the ball of 

 the eye. M. Magendie, 1 on the other hand, denies the existence of this 

 canal; and asserts that the tarsal cartilages do not touch by a rounded 

 edge, but by an inner plane surface. If we were to grant the existence 

 of this canal, it could only aid us in our explanation of the course of 

 the tears during sleep. In the waking state, they are not ordinarily 

 secreted in such quantity as to require that much should pass to the 

 puncta; the movements of nictation spreading them over the surface 

 of the eye, whence they are partly absorbed, and the rest, perhaps, 

 evaporated. Under extraordinary circumstances, however, the gland 

 increases its secretion so much, that the tears not only pass freely 

 through the lachrymal ducts into the nose, but flow over the lower 

 eyelid. The epiphora or watery eye, caused by obstruction of these 

 ducts, also proves that a certain quantity of the secretion must always 

 be passing into the puncta. The physical arrangement of the eyelids 

 and tunica conjunctiva is doubtless the cause of their course in this 

 direction. 



It has been gratuitously supposed by some, that the humour of 

 Meibomius prevents the tears from reaching the outer surface of the 

 lower eyelid, by acting like a layer of oil on the margin of a vessel 

 filled with water. A similar function has been assigned to the secre- 

 tion of the caruncula lachrymalis. Both these fluids, however, are 

 probably inservient to other ends. They are readily miscible with 

 water; become consequently dissolved in the tears, and, with the assist- 

 ance of the fluid secreted by the tunica conjunctiva, aid the movements 

 of the eyelids over the ball of the eye, and keep the tarsal margins 

 and their appendages in the condition requisite for the due perform- 

 ance of their functions. 



The action of the puncta themselves in admitting the tears has re- 

 ceived different explanations. M. Adelon 2 regards it as organic and 

 vital. We ought, however, in all cases, to have recourse to this mode 

 of accounting for phenomena as the ultima ratio; and the present ap- 

 pears to be a case in which it is singularly unnecessary. In many of 

 the results of absorption We are compelled to suppose, that a vital 

 operation must have been concerned in the process. Where, for 

 example, as in the case of the lymphatic vessels, we find the same fluid 

 circulating, whatever may have been the nature of the substances 

 whence it was obtained, the evidence, that a vital action of selection 



1 Precis, &c , edit, cit., i. 52. 



a Physiologie, 2de edit., p. 421, Paris, 1829. 



