THE FRONTIERS OF MEDICINE 



infection or another, whether he has been poisoned by 

 some chemical element, or whether his blood-forming 

 organs are functioning properly. But though it is known 

 that in the blood-forming and destroying system the bone 

 marrow and the spleen are especially significant and that 

 the liver and the gall bladder are seriously concerned, the 

 whole mechanism is so complicated and intricate that it 

 will require the best brains of thousands of scientists 

 for many years before it is fully understood. 



Blood Pressure. When a volume of fluid is forced through 

 a tube, the pressure of the fluid in the tube varies according 

 to the amount of pressure behind the fluid as it goes through 

 the tube, and the size of tube through which the fluid flows. 

 It varies also according to the ability of the tube to stretch. 

 If the pressure behind the fluid is temporarily lessened, the 

 pressure in the tube is lessened. If the tube is rigid, the 

 amount of pressure will increase over that of a tube which 

 can relax. These are some of the considerations which 

 govern the blood pressure of man. 



An increase in the amount of blood flowing through the 

 blood vessels will raise the blood pressure; a decrease in 

 the quantity will lower it. If the blood vessels become 

 hardened by arteriosclerosis, the blood pressure increases. 

 If the blood vessels are constricted by the use of certain 

 drugs, or by certain reactions in the nervous system, the 

 blood pressure increases. 



The amount of blood pressure can be measured in numer- 

 ous ways, including the method usually seen of putting a 

 cufl" around the arm, inflating it with air and reading the 

 amount of pressure on a mercury column or on a spring 

 device. Recently a machine has been developed that will 

 record with a moving finger or moving screen the varia- 

 tions in the blood pressure over a certain period. By listen- 

 ing with his stethoscope while the blood pressure is being 

 measured, the physician is able to determine two phases 



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