Chap. XII] GENERAL CIRCULATION OF BLOOD 221 



Method of determining blood pressure. — We can gain some 

 idea of the blood pressure by placing the fingers over a large 

 artery where the pulsation can be felt. If the vessel is tense and 

 hard to compress, it indicates high blood pressure. If, however, 

 it is easy to obliterate the pulsation by pressure of the fingers, 

 the blood pressure is low. Of late years several forms of apparatus 

 have been devised by which a more accurate knowledge can be 

 obtained. This apparatus is called a sphygmomanometer, and con- 

 sists of an air pump which is connected by means of an inverted 

 T (_L) shaped rubber tubing with an elastic bag covered on the 

 outside by a leather cuff, and a mercury manometer. 



The elastic bag covered with the cuff is buckled snugly around 

 the arm above the elbow. The bag is blown up by means of the air 

 pump and exerts pressure upon the brachial artery until no pulsa- 

 tion can be felt in the radial artery at the wrist. The amount of 

 pressure that is being exerted upon the arm is indicated by the 

 mercury manometer, and should be read the moment the pulse dis- 

 appears. This will give the maximum or systolic pressure. As the 

 pressure on the arm is lowered, the pulse reappears, and the lowest 

 position of the mercury gives the minimum or diastolic pressure. 



Normal degree of blood pressure. — The average normal degree 

 of blood pressure exerted on the walls of the brachial artery is 

 about 110 to 116 mm. systoHc; and 65 to 75 mm. diastoHc. 

 Pressure varies with age, and even in health is not constantly 

 the same. Increase of the force or rate of the heart-beat in- 

 creases the pressure. The effect of cold, also of certain drugs, 

 e.g. adrenalin, is to constrict the arteries and raise the blood press- 

 ure. On the other hand the effect of heat, and of some drugs, e.g. 

 amyl nitrite, is to dilate the arteries and lower the blood pressure. 



THE PULSE 



When the finger is placed on an artery, a sense of resistance 

 is felt, and this, resistance seems to be increased at intervals, 

 corresponding to the heart-beat, the wall of the artery at each 

 heart-beat being felt to rise up or expand under the finger. This 

 alternate contraction and expansion of the artery constitutes 

 the pulse ; and in certain arteries which lie near the surface this 

 pulse may be seen with the eye. Wlien the finger is placed on 

 a vein, very little resistance is felt; and, under ordinary cir- 



