NOURISHMENT AND PROTECTION OF THE EYE 515 



cornea moist, the evaporation keeping pace with the production. Under 

 certain circumstances there is excess, and tears are produced. 



TEAR FLUID consists chemically of an aqueous solution of sodium chloride 

 and carbonate containing mucus, albumen and debris. It is found to have 

 a bactericidal power which is lost if the fluid is boiled. Its functions are 

 to keep the surfaces of the conjunctiva and cornea moist, and to remove 

 foreign bodies and organisms. The secretion of tears is increased (1) by 

 irritants and foreign bodies coming in contact with the cornea, conjunc- 



COf+JUNCTIVA 



FIG. 256. Diagram to show origin and fate of tear fluid. 



tiva or lids ; (2) by irritation of the nasal mucous membrane ; (3) by power- 

 ful illumination of the eyes ; (4) By the incidence on the eye of infra-red 

 (heat) and ultra-violet (actinic) rays ; (5) under the influence of emotion. 

 When excessive tear formation occurs the fluid either escapes over the 

 front of the lids, or is drained away through the lachrymal duct into 

 the nasal sinus. Three theories have been advanced to explain the 

 latter: (1) syphoning, owing to the mouths of the ducts being at a 

 higher level than their exit into the nose; (2) capillarity, owing to the tendency 

 of the liquid to flow into the ducts through surface tension ; (3) active 

 removal by the act of blinking. It is not at the present time definitely known 

 how this occurs. Some say that on closing the eye the internal palpebral 

 ligament tends to be pulled on, and that this dilates and fills the lachrymal 

 sac ; others that the sac fills automatically through having been previously 

 emptied by the contraction of Homer's muscle. It is possible that both 

 processes occur. 



The eyes of some fish and nearly all birds are provided with a nictitating membrane, 

 a semi-transparent shutter which can be brought over the surface of the cornea. 

 In the fish its possible function is to prevent the irritation of fine sand particles when 

 swimming in rough water, without at the same time disturbing vision. In the case 

 of the bird it might be used (1) to moisten the cornea during flight without interrupting 



