THE SECRETION OF TEARS. 795 



edge of a glass vessel prevents the water in it from overflowing]. The tears are 

 conducted from the puncta through the duct, chiefly by a siphon action. Horner's 

 muscle (also known to Duvernoy, 1678) likewise aids, as every time the eyelids are 

 closed it pulls upon the posterior wall of the sac, and thus dilates the latter, so 

 that it aspirates tears into it (Henke). 



E. H. Weber and Hasner ascribe the aspiration of the tears to the diminution of the amount 

 of air in the nasal cavities during inspiration. Arlt asserts that the tear sac is compressed by 

 the contraction of the orbicularis muscle, so that the tears must be forced towards the nose. 

 Lastly, Stellwag supposes that when the eyelids are closed the tears are simply pressed into the 

 puncta, while Gad denies that there is any kind of pumping mechanism in the nasal canal. 

 Landois points out that the tear ducts are surrounded by a plexus of veins, which according to 

 their state of distension may influence the size of these tubes. 



The secretion of tears takes place only by direct stimulation of the lachrymal 

 nerve ( 347, I., 2), subcutaneous malar ( 347, II., 2), and cervical sympathetic 

 ( 356, A, 6), which have been called secretory nerves. Secretion may also be 

 excited refiexly (p. 591) by stimulation of the nasal mucous membrane only on the 

 same side (Herzenstein). The ordinary secretion in the waking condition is really 

 a reflex secretion produced by the stimulation of the anterior surface of the bulb 

 by the air, or by the evaporation of tears. A very bright light also causes a 

 reflex secretion of tears, the optic being the afferent nerve. The centre in the 

 rabbit does not extend forward beyond the origin of the fifth nerve, but it extends 

 downwards to the fifth vertebra (JSckhard). During sleep all these factors are 

 absent, and there is no secretion. Histological changes. Reichel found that in 

 the active gland (after injection of pilocarpin) the secretory cells became granular, 

 turbid, and smaller, while the outlines of the cells became less distinct, and the 

 nuclei spheroidal. In the resting gland, the cells are bright and slightly granular 

 with irregular nuclei. Intense stimulation by light acting on the optic nerve causes 

 a reflex secretion of tears. The flow of tears accompanying certain violent emotions, 

 and even hearty laughing, is still unexplained. During coughing and vomiting 

 the secretion of tears is increased partly refiexly, and partly by the outflow being 

 prevented by the expiratory pressure. 



Function. The tears moisten the bulb, protect it from drying, and float away 

 small particles, being aided in this by the closure of the eyelids. Atropin 

 diminishes the tears (Mogaard). 



Composition. The tears are alkaline, saline to taste, and represent a " serous " 

 secretion. Water 98*1 to 99 ; 1*46 organic substances (0*1 albumin and mucin, 

 O'l epithelium) ; 0*4 to 0-8 salts (especially NaCl) 



[Action of Drugs. Essential volatile oils and eserin increase the secretion of tears, atropin 

 arrests it, while eserin antagonises the effect of atropin and causes an increased secretion.] 



405. COMPAEATIVE HISTOEICAL. Comparative. The simplest form of visual appa- 

 ratus is represented by aggregations of pigment-cells in the outer coverings of the body, which 

 are in connection with the termination of afferent nerves. The pigment absorbs the rays of 

 light, and in virtue of the light-ether discharges kinetic energy, which excites the terminations 

 of the nervous apparatus. Collections of pigment-cells, with nerve-fibres attached, and pro- 

 vided with a clear refractive body, occur on the margin of the bell of the higher medusae, while 

 the lower forms have only aggregations of pigment on the bases of their tentacles. Also, in 

 many lower worms there are pigment spots near the brain. In others, the pigment lies as a 

 covering round the terminations of the nerves, which occur as "crystalline rods" or "crystal- 

 line spheres." In parasitic worms, the visual apparatus is absent. In star-fishes, the eyes are 

 at the tips of the arms, and consist of a spherical crystal organ surrounded with pigment, with 

 a nerve going to it. In all other echinodermata there are only accumulations of pigment. 

 Amongst the annulosa there are several grades of visual apparatus (1) Without a cornea, there 

 may be only one crystal sphere (nervous end-organ) near the brain, as in the young of the crab ; 

 or there may be several crystal spheres forming a compound eye, as in the lower crabs. (2) 

 With a cornea, consisting of a lenticular body formed from the chitin of the outer integument, 

 the eye itself may be simple, merely consisting of one crystal rod, or it may be compound. The 

 compound eye consists of only one large lenticular cornea, common to all the crystal rods, as in 

 the spiders ; or each crystal rod has a special lenticular cornea for itself. The numerous rods 



