400 THE LACHRYMAL APPARATUS. 



that organ from undue intensity of light, or preserve it from the ingress 

 of drops of sweat. They aid greatly in the expression of mental emo- 

 tions, but perhaps should rather be looked upon as among the remaining 

 vestiges of the hairy tegument which affords a protection to the entire 

 skin of other mammals below man in the animal series. The eyelids 

 may be described as a pair of valves, the upper one having a much great- 

 er latitude of motion than the lower. Their use is to afford protection 

 to the eye by closing entirely over it, more particularly during sleep ; to 

 keep its optical surface moist and free from dust by their winking mo- 

 tion. They are brought into action by the contact of air or of irritating 

 particles, through the fibres of the fifth and facial nerves, or by the agency 

 of light upon the retina. The edges of the lids are furnished with rows 

 of curved hairs, the eyelashes, which add greatly to the protection of the 

 delicate organ beneath, while permitting vision to take place to a certain 

 extent. Opening upon the edges of the eyelids are the foramina of the 

 Meibomian glands, in the upper lid there being about thirty, in the low- 

 er somewhat fewer. The glands themselves are imbedded on the in- 

 ternal surface of the cartilage of the lids, and afford an oily secretion, 

 which discharges the double duty of preventing adhesion of the lids, and, 

 by its relation of capillary attraction, hindering the overflow of the water 

 which moistens the eye upon the cheek. 



Of the lachrymal apparatus, it may be said that in the same manner 

 The lachrymal tna * we breathe upon a spectacle glass and wipe it that its 

 apparatus. surface may be perfectly clean, so it is necessary for the op- 

 tical action of the cornea that its surface should be constantly washed, 

 and even more so, for its lamellated- structure is such that, if it be not 

 kept constantly damp, it loses much of its transparency. This therefore 

 renders it necessary that there should be a mechanism for the supply of 

 water, another for spreading that water uniformly over the surface of the 

 cornea, and a waste-pipe for carrying any surplus away. The lachrymal 

 gland discharges the first of these duties. It is situated in the upper 

 and outer angle of the orbit ; its secretion, which is a bitter and some- 

 what saline water, is brought to the surface of the conjunctiva by eight or 

 ten little ducts arranged in a row for the purpose of equalizing their dis- 

 charge. The spreading of this fluid over the eye, and the simultaneous 

 wiping of the surface, is accomplished by the eyelids. Usually the wa- 

 ter that has been employed is dissipated by evaporation into the air; but 

 if, by reason of meteorological circumstances, such as the dampness of 

 the atmosphere, or by the supply being too abundant, there should arise 

 an excess, it is carried off through two minute orifices which are upon the 

 edge of the eyelids, the puncta lachrymalia. These draw off any collection 

 of water that may have accumulated in the lachrymal lake, and, carrying 

 it into the lachrymal sac, discharge it through the nasal duct into the 



