AND BLOOD PRESSURE 141 



the brain or bone marrow the amount of blood can be enormously 

 increased by the transference of tissue fluid and tissue substance 

 from the tissue cells into the blood channels. 



The way in which increased tension may arise can be illustrated 

 by a consideration of the cause of glaucoma of the eye. 



In glaucoma the cribriform ligament is sclerosed, and by wall- 

 ing in Schlemm's canal prevents the free circulation of the aqueous 

 humour which is necessary for the proper metabolism of the eye. 

 When the cribriform ligament is sclerosed the aqueous has to get 

 away mostly by the veins of the iris through the iris crypts, and 

 thus it comes about that atropine induces an attack of glaucoma 

 by dilating the pupil and closing the orifices of the crypts of the 

 iris. Iridectomy, on the other hand, relieves glaucoma by opening 

 up new channels for escape of the aqueous, as the surface of 

 the iris never scars up, but remains open and unaltered after it 

 has been cut (Thomson Henderson). 



The oedema of the tissues of the eye which occurs in glaucoma 

 may be ascribed to the deficient circulation of aqueous, leading to 

 altered metabolism and increase of osmotic pressure. The conges- 

 tion of the blood vessels must be also set down to the irritant 

 effect of the products of disordered metabolism. A rise of 

 general vascular pressure may, it is said, precipitate an acute 

 attack of glaucoma ; probably by increasing the secretion of 

 aqueous ; this finds room at the expense of the veins which 

 are narrowed, i.e. the absorbing surface ; the excess of aqueous 

 cannot escape again when the general pressure falls, and thus the 

 tension of the eyeball remains heightened, the circulation in the 

 eye lessened and the mischief increased. 



The conditions which hold good for the eye and brain also 

 apply in part to the limbs enclosed by the skin, and to the other 

 organs, such as the kidney, which are enclosed in capsules. The 

 writer and M. Flack estimated the capillary pressure in man by 

 pushing a hollow needle into the subcutaneous tissue of the arm 

 or leg, and connecting this with a glass tube containing an air 

 bubble, and the tube with an Hg manometer and a pressure bottle. 

 The air bubble indicates when the pressure is sufficient to over- 

 come the capillary pressure and drive the water in, and as the 

 subject feels the smart of the water, he can give a confirmatory 

 signal. The observations made in this way showed that a pressure 

 of 10 to 20 mm. H 2 was sufficient to overcome the capillary 



