430 PHYSIOLOGY CHAP. 



unilateral stimulation of the trigeminal nerve usually produces a 

 flow of tears only upon the same side (Wilbrand and JSanger). 



From the lachrymal secretion reflexly produced by peripheral 

 stimuli we must distinguish that evoked by central impulses, viz. 

 tears in yawning or crying. 



The origin of the nerves to the lachrymal glands and the 

 centre of reflex or direct lachrymal secretion is not yet known. 

 According to the investigations of Campos (1877) the rarnus 

 lachrimalis of the trigeminal nerve contains secretory fibres that do 

 not come from the facial nerve; but Goldscheider (1895), Uthoff 

 (1886) and others noted that in total paralysis of the facial the 

 flow of tears in weeping is absent on the paralysed side. Accord- 

 ing to Eckhard (1867) the centre for lachrymal secretion lies in 

 the upper part of the medulla oblongata ; according to Seek it is 

 situated between the 6th cervical segment and the lower end of 

 the spinal root of the trigeminus; according to Bechterew and 

 Mislawsky in the corpora quadrigemina. 



How tears flow through the lachrymal passages is not yet 

 settled. Haller first stated that the tear-points took up the tear 

 by capillary attraction and conducted them to the lachrymal sac, 

 thus contradicting the hypothesis of Petit, that the lachrymal 

 apparatus acts as a siphon. Hounault, E. H. Weber, and v. 

 Hasner held that the inspiratory current of air causes suction of 

 the tears collected in the lachrymal lake. Against this hypothesis 

 we have the fact demonstrated by E. H. Weber that on intro- 

 ducing a small manometer into one of the tear-ducts no fall in the 

 level of the fluid can be observed during inspiration. It is certain 

 that winking accelerates the flow of tears into the lachrymal 

 passages. If a fluid covered with aniline be introduced into the 

 lachrymal lake, it is rapidly absorbed if the eyes are closed 

 repeatedly, but very slowly if the lids are kept open. It is there- 

 fore obvious that the orbicularis muscle and Homer's muscle 

 exert some action on the penetration of tears into the lachrymal 

 passages. But how do they do so? According to Eichter and 

 Schmidt, closing the eyes dilates the lachrymal sac and allows it 

 to fill ; Arlt and Moll, on the contrary, hold that this compresses, 

 and thus helps to empty it. Henke admitted both these interpre- 

 tations, and assumed that during the closure of the lid the 

 lachrymal sac is dilated by contraction of the palpebral portion of 

 the orbicularis, so that the tears are aspirated, and during opening 

 of the lid the lachrymal or Homer's part of this muscle contracts, 

 and the tear-sac is compressed. Gad contiadicted this last state- 

 ment of Henke, and assumed that contraction of the sac is due to 

 its elasticity. 



Scimeni, on introducing a small tube of fluid through a fistula 

 in the lachrymal sac, found that the fluid was rapidly aspirated 

 into the sac on closing the eyelids, and slowly returned to the tube 



