138 .THE VASCULAR SYSTEM 



of the brain) by inserting a tube through the lamina of the axis. 

 The brain pressure, the cerebral venous pressure, and the cerebro- 

 spinal fluid pressure are on and the same. 



The writer has also measured the capillary-venous pressure in 

 the eyeball by the same method, inserting a hollow needle into 

 the aqueous. The globe formed by the corneo-sclera is analogous 

 to the rigid case formed by the skull because its internal capacity 

 is strictly limited, and cannot be increased. The intra-ocular 

 pressure, like the intra -cranial, is purely circulatory in origin and 

 represents the capillary- venous pressure within the eye. The venous 

 sinus of the canal of Schlemm, according to Thomson Henderson, 

 is formed of tributaries from the whole anterior portion of the 

 uveal tract, and it thus offers the lowest pressure with which the 

 aqueous humour comes into direct contact, just as the pial veins, 

 where they pass into the sinuses, offer the lowest pressure to the 

 cerebro-spinal fluid. The intra-ocular fluid, just like the intra- 

 cranial, transmits pressure equally in all directions, and therefore 

 when its exit is diminished the tension rises, and the whole of 

 the elastic intra-vascular system tends to be converted into a 

 rigid one. 



Koster has proved experimentally the unyielding nature and 

 rigidity of the globe under conditions of increased intra-ocular 

 tension. If the internal pressure were raised from 19 to 70 mm. Hg, 

 the increase in volume of the globe was only T? nn7 of the original 

 volume. 



In the globe of the eye the capillary-venous pressure is the intra- 

 ocular pressure. The intra-ocular pressure in fact equals that in the 

 venous sinus of Schlemm's canal, just as in the skull the pressure 

 of the cerebro-spinal fluid is equal to that in the pial veins. 



Supposing the intra-ocular tension, owing to the increased 

 secretion of the ciliary process, rise above the pressure within 

 Schlemm's canal, then the pressure of the aqueous will obliterate 

 the wall of the sinus until the venous pressure rises within it to 

 that in the aqueous. Similarly, supposing by increased secretion 

 of the choroidal fringes in the brain the pressure of the cerebro- 

 spinal fluid rise above that in the pial veins, then these would be 

 obliterated until the pressure of the blood within them rise to that 

 of the cerebro-spinal fluid. Actual measurements show that the 

 pressure of the cerebro-spinal fluid and of the cerebral venous 

 pressure are the same. 



