1912.] Relation between Capillary Pressure and Secretion. 451 



vascular system of the eye becomes a rigid system, the arterial pressure per- 

 taining alike to arteries, capillaries, and veins. The outflow of blood from 

 the veins of the eye ceases when the fluid pressure is made greater than the 

 arterial pressure. It is obvious in this case that the pressure of the fluid 

 outside and the blood inside the capillaries must be one and the same. The 

 conditions are similar to those which are established in the brain when an 

 artery ruptures and blood escapes at arterial pressure into the cranial cavity. 

 The veins are compressed and the blood within them expressed until the 

 pressure in the exsanguined brain rises to arterial pressure. When the 

 hemorrhage ceases and the blood clots the pressure in the brain still remains 

 high because of the volume of the clot, which takes up the room formerly 

 occupied by blood in the cerebral vessels. The pressure which rules in the 

 exsanguined part is the arterial pressure. The cause of the cerebral pressure 

 is thus the arterial pressure, but the height of that pressure is determined 

 not only by the force of the heart beat but also by the volume of fluid 

 extruded within the skull cavity. The difference between the brain and the 

 eye is that the secretory pressure within the eye is maintained at a much 

 higher level. 



The cerebro-spinal fluid is secreted at a pressure about the same as the 

 general venous pressure, and saline injected into the intracranial cavity at a 

 higher pressure is rapidly absorbed ; the capillaries and veins afford the 

 pathway for absorption. If saline be run into the subdural space at a high 

 pressure the outflow from the cerebral veins ceases when the arterial pressure 

 is exceeded by that of the fluid forced into the skull cavity. The same, we 

 show, holds good for the eye. 



In the case of the salivary gland, enclosed as it is in a capsule, the secretory 

 pressure regulates the circulatory pressure. When the secretion is 

 obstructed, the secretory pressure rises, the distension of the alveoli narrows 

 the veins and the capillary venous pressure and the tenseness of the gland 

 rise pari passu with the secretory pressure ; the arteries are dilated, the 

 whole circulatory system approximates to a rigid system, with almost the 

 full arterial pressure ruling throughout, giving a greatly accelerated outflow 

 from the veins. .Finally the salivary pressure may rise much above the 

 arterial pressure, but even then the circulation is not strangulated, because the 

 membrame propria? limit the expansion of the alveoli. 



In the brain and in the eye there exist no such menibranas proprise, which 

 can permit a difference of pressure between the secreted fluid and the blood. 

 The ciliary processes are soft protoplasmic structures hanging in a bath of 

 aqueous. The pressure of the blood within the capillaries and of the aqueous 

 without must be one and the same. When fluid is forced in at a higher 



