456 ORGANS OF THE SENSES. 



The secretion of the tears is constant ; it is increased by 

 moral causes, or by reflex irritations of the cornea, but some- 

 times also from the nasal mucous surface or from the retina. 

 If the cornea be irritated by any foreign body, a hypersecre- 

 tion of tears follows from the irritating nature of the substance 

 that is dissolved or carried away. This secretion is produced 

 by a reflex phenomenon exactly resembling that which 

 governs the secretion of the saliva. The centrifugal nerve 

 of this reflex action is the lachrymal nerve (a branch of the 

 ophthalmic nerve coming from the fifth pair). The hyper- 

 secretion of tears which, by a reflex action, follows the irrita- 

 tion of many of the cranial nerves (the frontal, the nasal, the 

 lingual, the glosso-pharyngeal, and the pneumo-gastric nerves), 

 discontinues after section of the lachrymal nerve. According 

 to Demtschenko, irritation of the great sympathetic nerve 

 also causes lachrymal hypersecretion, in the same way as we 

 have seen that it occasions the secretion of saliva (see p. 218) ; 

 in this case, however, the tears are of a peculiar nature, re- 

 sembling the saliva under similar circumstances ; the secretion 

 is thick and cloudy, while that following irritation of the tri- 

 facial, is limpid and transparent 1 (compare with this what is 

 said on p. 219). 



The tears evaporate to a certain extent, but a portion 

 always remains ; this portion is prevented from flowing over 

 the eyelids and running down the cheeks, by means of the 

 fatty secretion of the meibomian glands (see sebaceous glands 

 and their functions) ; these latter are found on the edges of 

 the eyelids, and are more numerous in the inner angle of the 

 eye. From here the tears pass (Fig. 123), by the puncta 

 lachrymalia, successively through the lachrymal canals, the 

 lachrymal sack, and the nasal duct, until they reach the 

 nasal chambers, at the anterior portion of the inferior meatus. 

 Many reasons, some of more weight than others, have been 

 suggested, in order to account for the passage of the lachry- 

 mal fluid through this series of tubes ; some have supposed 

 it to be produced by capillarity, but this physical force, by 

 means of which a fluid penetrates a small empty tube, is 

 rather a hinderance than an. aid to movement, if the tube is 

 full. 1 This is likewise true of the comparison of the lachry- 



1 Demtschenko, " Zur Innervation der Thranendriise." Pflii- 

 ger's Archiv., Sept., 1872. 



1 See Foltz, " Des Voies Lacrymales." " Journal de Physio- 

 logic," by Brown-Sequard. Vol. V., 1862. 



