1912.] Relation between Capillary Pressure and Secretion. 457 



from in front of the lens to the circumlental region." This, we believe, is 

 the purpose of the structural arrangements examined by Arthur Thomson. 

 The transference of fluid is essential to the act of accommodation. 



In text-book discussions on the mechanism of accommodation, little or no 

 attention is paid to the behaviour of the aqueous at the moment when the 

 anterior surface of the lens changes its shape. The ciliary muscle is 

 described as fixed at the junction of the cornea and sclerotic and inserted 

 into the choroid ; it is supposed to pull up the choroid so as to relax the 

 tension of the suspensory ligament when the lens by virtue of its elasticity 

 becomes more convex. From whence, we may ask, does the lens obtain the 

 " virtue of its elasticity " ? An excised lens has an almost fluid exterior and 

 a much stiffer glue-like core. The tension of the choroid is due to the intra- 

 ocular pressure, this depends on the secretory pressure of the aqueous, which 

 in its turn determines the circulatory pressure in the eyeball. The pressure 

 in the aqueous, in the vitreous, in the lens, and in all the capillary-venous 

 networks within the sclerotic-corneal envelope is one and the same. While 

 the vitreous is enclosed in the hyaloid membrane, the membranes of 

 Descement, of Bruch, and the basement membrane of the iris and ciliary 

 processes form one continuous membrane, which, we believe, acts like the 

 membranes propriae of the alveoli of the salivary gland — a kind of semi- 

 permeable membrane, permitting and confining the secretion of aqueous. 

 The fluids of the eye distribute the pressure equally in all directions. The 

 manometric observations of Hess and Heine show that no change of intra- 

 ocular pressure accompanies the act of accommodation. ISTo change in 

 the fulness of the retinal vessels has been observed in the eye of man on 

 accommodation. 



On the other hand, Arthur Thomson cites observations made by Ulbrich 

 on a case of partial coloboma of the iris, where the aperture was closed by 

 a delicate membrane which served as a natural and very sensitive manometer. 

 Although dilatation and contraction of the pupil under the action of light 

 did not alter the position of the membrane, yet during accommodation the 

 membrane sank, the extent to which it receded being roughly proportional 

 to the amount of accommodation. Under eserine this reaction was intensified 

 whilst atropine, on the contrary, annulled it entirely. Now, in order that the 

 lens may become convex forwards it is necessary that fluid should be transferred 

 from the anterior chamber in front to the region surrounding the circum- 

 ference of the lens. Helmholtz recognised this, and supposed that the angle 

 of the anterior chamber was deepened to make room for the displaced aqueous. 

 Thomson Henderson controverts this on the evidence derived from the 

 structure of eyeballs which have been properly fixed, and points out that 



