. Diseases of children. h is graded from a Hght to a deep reddish hue,the two sides are so adjusted that they will be of the sameshade. The percentage is read from a small scale attachedto the shding prism. Normal blood will read 100 per cent,on this instrument. The specific gravity of the blood is obtained byplacing a drop of blood in a fluid mixture of knownspecific gravity. When the drop remains stationary thespecific gravity is the same as the mixture. 192 DISEASES OF CHILDREN FOR NURSES Blood-pressure.—This is taken by means of anapparatus (a sphygmomanometer) which registers inmilHmeters
. Diseases of children. h is graded from a Hght to a deep reddish hue,the two sides are so adjusted that they will be of the sameshade. The percentage is read from a small scale attachedto the shding prism. Normal blood will read 100 per cent,on this instrument. The specific gravity of the blood is obtained byplacing a drop of blood in a fluid mixture of knownspecific gravity. When the drop remains stationary thespecific gravity is the same as the mixture. 192 DISEASES OF CHILDREN FOR NURSES Blood-pressure.—This is taken by means of anapparatus (a sphygmomanometer) which registers inmilHmeters of mercury the amount of pressure that ispresent in the arteries during (i) the passage of the pulsewave (systoHc pressure), (2) the actual pressure in thearteries during diastole (diastolic pressure). A rubber bag contained in an arm band is placed aboutthe arm above the elbow and air is pumped into this baguntil the pressure is sufhcient to compress the artery atthat point. This, of course, stops the pulse. The air. Fig. 50.—Rogers sphygmomanometer (Morrow). is then allowed to escape until the pulse can be felt atthe wrist or heard through the stethoscope placed overthe brachial artery below the encircling band. At thispoint the systolic pressure is read off. The diastolicpressure is obtained by allowing the air to escape untilthe pulsations are no longer audible through the stetho-scope ; at the point where the sound ceases we read off thediastolic pressure. The difference between the two readings is called thepulse pressure. The normal pressure varies with the age. From three DISEASES OF THE CIRCULATORY SYSTEM 193 to thirteen the normal systolic pressure is fairly con-stant between 90 and 103 At the age of pubertyit rises rather rapidly toward the adult figures, whichnormally should be between 115 and 130 In infants under three years of age it is impracticableto take blood-pressure. Normal blood contains 5,000,000 red blood corpusclesto a cubic millimeter. At
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