vin OCULAIi MOVEMENTS 431 



on opening them. It is not apparent why, under normal con- 

 ditions during the dilatation of the sac produced by the contraction 

 of the orbicularis, the tears should be absorbed by the narrow tear- 

 points rather than sucked into the sac from the nasal passage, and 

 why during its contraction they should not again flow back through 

 the tear-points. In partial explanation of this fact, it is well to 

 remember that the nasal extremity of the lachrymal canal is closed 

 by a valve described by Hasner, which prevents a back-flow of 

 tears, but permits them to pass into the nasal cavity. 



XI. In conclusion we must glance at the properties and origin 

 of the humours of the eye, as well as the intraocular pressure to 

 which they give rise. 



The aqueous humour is a clear, colourless fluid, alkaline in re- 

 action, of a specific gravity that varies between 1-0034 and 1*0060, 

 and contains about 0'S6 per cent solids, 0'045 per cent of which 

 consist of protein albumin and globulin. The normal fluid, of 

 the eye contains no tibrinogen, but sugar up to 0'03-0'05 per cent. 

 Its osmotic pressure, according to the researches of several recent 

 workers, is higher than that of blood. If the anterior chamber of 

 the eye is artificially emptied, it fills again rapidly with fresh fluid, 

 which is not, however, exactly the same as the normal, because it 

 contains more protein and also fibrinogen, so that it may coagulate 

 spontaneously. But six hours after evacuation it again has all 

 the properties of normal aqueous humour. If, after the puncture, 

 the sympathetic be stimulated, or a solution of suprarenal extract 

 introduced into the conjunctival sac both of which produce 

 vascular constriction these changes in the property of the newly- 

 formed aqueous humour do not occur. 



The origin of the aqueous humour has been and still is much 

 discussed. According to Leber and many others, the ciliary 

 processes are the anatomical structures that produce it. The 

 arguments in favour of this theory are as follows : 



(a) Extirpation of the ciliary body along with the iris causes 

 total disappearance of the intraocular eye-fluids (Leber, Deutsch- 

 mann) ; on the other hand, congenital absence or complete 

 removal of the iris does not hinder the formation of this fluid. 



(b) If, after connecting the posterior chamber of the eye 

 through the pupil with a manometer, the pressure of the latter is 

 made equal to that inside the eye, it is found that pressure rises. 

 If the aqueous humour is allowed to escape, by perforation of the 

 cornea, so that the iris is pushed forward against the posterior 

 corneal surface, it will be seen that this condition remains 

 permanent ; so too, in cases in which the pupil is adherent to the 

 lens capsule, the iris is pushed forward. These facts are easily 

 explained on the assumption that the aqueous humour comes 

 from the posterior chamber of the eye, i.e. is produced by the 

 ciliary processes. 



