432 PHYSIOLOGY CHAP. 



Ehrlich disputes this theory. In 1882, he observed in the 

 rabbit that after injection of fluorescin a greenish, fluorescent 

 line appeared at the margin of the iris and spread towards its 

 base. No flow of fluorescin from the pupil can be seen previous 

 to the appearance of this line. From this Ehrlich concluded that 

 the aqueous humour is produced, not from the ciliary bodies, but 

 from the anterior surface of the iris. On the other hand, this is 

 also observed on dead animals, when fluorescin is injected into 

 the veins (Ehrenthal), and sometimes, when the ciliary bodies 

 are hyperaemic, fluorescin flows from the pupil before Ehrlich's 

 line appears (Nicati). 



^Another theory put forward by Hamburger (1899) assumes 

 that the aqueous humour under normal conditions is produced by 

 the anterior surface of the iris, but that when the flow is exag- 

 gerated the ciliary processes also share in its formation. He 

 adduces in support of his theory the observation that aqueous 

 humour is already present in the foetal eye, even when the 

 membrane that occludes the aperture of the pupil is still present. 



The various authors again disagree as to the nature of the 

 physiological process that gives rise to the formation of the 

 aqueous humour. According to Leber it is due to filtration from 

 the blood-vessels. Against this view, however, Hamburger and 

 Bottazzi point out correctly that there are marked differences 

 between the physical and chemical properties of the aqueous 

 humour and of blood plasma, which are inexplicable if we assume 

 it to be formed by a simple process of filtration. It has, in fact, 

 a higher osmotic pressure and electrical conductivity than serum, 

 besides considerable differences in its chemical constitution as 

 compared with serum. Accordingly, we must assume that the 

 fluid of the eye is formed by a process analogous to secretion. 

 In support of this theory is the fact repeatedly observed by many 

 authors that lesions of the ocular nerves (sympathetic and 

 trigeminal) produce changes in the constitution of the aqueous 

 humour. The question next arises whether we are to assume 

 that there is a genuine secretory process effected by a differentiated 

 glandular epithelium, or by a process similar to that concerned in 

 the formation of lymph (supra, Vol. I.), that is, by the activity of 

 the endothelial elements of the walls of the blood capillaries, as 

 suggested by Angelucci (1902). 



Once formed, the aqueous humour does not remain stationary 

 inside the chambers of the eye, but as it is formed a portion of 

 it is reabsorbed by other parts of the eye, and returned to the 

 lymph spaces. This hypothesis is based mainly upon the 

 observation that the renewal of the humour takes place a com- 

 paratively short time after it is evacuated. Obviously, however, 

 no logical conclusions can be deduced from this in regard to the 

 normal circulation of the fluid. The most important objection 



