vin OCULAE MOVEMENTS 429 



similar to those described in speaking of the salivary glands 

 (Vol. II.). The epithelium of the secretory ducts has no rod 

 structures or cilia. The ducts of the two larger glands, 10 to 12 

 in number, pass obliquely through the mucous membrane and 

 open in the upper and outer part of the conjunctival fornix. 



The chemical composition of tears, i.e. the mixed secretion of 

 the lachrymal glands as a whole, was first analysed by Fourcroy 

 and Vauquelin (1846). It is a very watery fluid, alkaline in 

 reaction, strongly saline in taste ; so far as is known, it contains 

 no enzymes, but has a little albumin in solution, with mucus, 

 sodium chloride, and traces of sodium carbonate, alkaline 

 phosphates, and alkaline earthy salts. Under the microscope it 

 is seen to contain epithelial cells thrown off from the lachrymal 

 ducts and the conjunctival surface, and fat droplets secreted by 

 the Meibomian glands. The daily output of secretion is estimated 

 at about 3 grins, for each eye. 



The lachrymal secretion is continuous, but increases whenever 

 certain stimuli act reflexly upon the lachrymal glands. The 

 lachrymal fluid spreads from the upper and outer part of the 

 conjunctival sac by capillarity, over the whole of the anterior 

 surface of the eye, between the palpebral and bulbar layers of the 

 conjunctiva, to the inner angle of the eye the so-called lachrymal 

 lake. During winking the lachrymal fluid on the corneal surface 

 is renewed, and its passage towards the lachrymal lake facilitated ; 

 the overflow passes into the so-called lachrymal points and 

 through the ducts into the lachrymal sac, whence it eventually 

 flows into the nasal canal. 



When the secretion is not excessive, as it is in crying, the 

 tears do not pass beyond the free margins of the lids, because they 

 are kept back there by the sebaceous secretion of the Meibomian 

 glands. Schirmer (1902) observed on 50 individuals in whom he 

 had excised the lachrymal sac that the tears even then seldom or 

 never overflowed the edge of the lower lid, unless an increase of 

 secretion was produced artificially. It is therefore probable that 

 under normal conditions there is no drainage through the lachrymal 

 canals, evaporation from the free surface of the eye and con- 

 junctival absorption being sufficient to remove the tears. It is 

 only when the flow of tears exceeds the normal that they collect 

 in the lachrymal lake. 



Lachrymal secretion, like the movements of the lids, is 

 normally a reflex act produced by peripheral stimuli. These may 

 act on the optic nerve (bright light) or on the trigeminal (drying 

 by evaporation or by currents of air, chemical stimulation by irri- 

 tating gas, mechanical stimulation by particles of dust or other 

 foreign bodies). Chemical or mechanical irritation of the nasal 

 mucosa also produces a reflex lachrymal secretion. Unilateral 

 stimulation of the optic nerve causes a flow of tears in both eyes, 



