THE LACRIMAL SYSTEM 41 



but numerous experiments have shown that this is not the case. Though 

 many factors have been tested for their effect or lack of effect upon the 

 acceleration or inhibition of the rhythmical blinking of the lids, the im- 

 mediate cause of it remains quite unknown. 



The Lacrimal System — The tear fluid, which can be thought of as 

 the land animal's substitute for an ocean, is produced continuously in 

 small amounts (less than 1 cc. per day in the absence of irritation) by 

 the lacrimal gland. This compound tubular gland lies against the su- 

 perior temporal quadrant of the eyeball in the anterior part of the orbit, 

 propped forward by the orbital fat (Fig. 17). Its dozen ducts open 

 mostly far up under the upper lid. Like the lids themselves, the entire 

 lacrimal apparatus is lacking in fishes, where it is not needed, and is 

 much reduced in those aquatic forms which have had terrestrial ancestry. 

 The tears are mixed with mucus secreted by scattered cells in the con- 

 junctiva, and most of this fluid is disposed of by evaporation. Any 

 excess, upon irritation of the eye or in mild emotional states, drains 

 through the two punctae — chiefly the lower — into a pair of canaliculi 

 which converge and enter the 'lacrimal sac'. This is a dilatation of the 

 upper end of the lacrimal duct, a membranous canal which runs vertically 

 downward, through the bony substance of the skull, to empty into the 

 nasal cavity. This connection leads to our being able to taste the salty 

 tears in the back of the mouth when we weep. There are a number of 

 so-called valves in the tear-drainage system, and its action is rather com- 

 plicated; but the essential factor in emptying the lacus is a pumping 

 action by the orbicularis oculi upon the adjacent lacrimal sac. This 

 makes it possible to conceal emotion and sometimes to forestall weeping 

 (the spilling of excessive tear fluid onto the cheeks) by rapid blinking. 

 The primary use of the tears is to clean and wet the cornea. Their 

 overproduction upon irritation is often entirely effective in washing away 

 the source of irritation. The fluid contains enough sugar and protein 

 to be of value in the nutrition of the corneal epithelium, which is able 

 to absorb proteins. There is some evidence that it is the sole source of 

 that nutrition. Moreover, the tears are bactericidal to a not unimportant 

 extent due to the presence in them of a special antiseptic ferment, 'lyso- 

 zyme'. The most conspicuous thing about the lacrimal system, however, 

 — psychical (emotional) weeping — is strictly peculiar to the human 

 animal and to some species of bears, and serves no physiological pur- 

 pose whatever. It§ value is wholly psychological and economic — as every 

 woman knows! 



