THE CIRCULATION 107 



tions in pressure and note that each is made up of a number of cardiac 

 variations. Does the tracing show Traube-Hering curves, which are 

 rhythmic waves including several respiratory variations? Ascertain the 

 height of the pressure. 



Annotation It is best to establish a certain degree of pressure in the manometer 

 before the blood is actually allowed to act against the mercury. In a cat or dog we 

 may expect to obtain a pressure equalling 120 to 130 mm. Hg. Thus, if we estab- 

 lish a pressure of 100 mm. Hg. beforehand, only a small quantity of blood need pass 

 into the glass cannula to produce the additional pressure. On the other hand, if 

 the recording needle is allowed to remain at zero to begin with, the aforesaid 

 degree of pressure can only be produced by the passage of a considerable portion of 

 the blood into the connecting tubes. In small animals this would give rise to a 

 material decrease in pressure, and would greatly favor the occurrence of coagulation 

 in the connecting tube. 



To obtain the blood-pressure, measure the distance between the zero-line and 

 the midline of the oscillations recorded by the manometer (H). Employ the formula: 



Obviously, the recording needle registers merely the upward movement of the 

 mercury in the distal limb of the manometer. It must be remembered, however, 

 that the mercury in the central limb moves at this time an equal distance in a down- 

 ward direction, and hence the distance between the zero-line and the oscillations in 

 pressure, as registered by the needle, must be multiplied by 2. In addition, the cor- 



rection --- must be made, because the blood acts against mercury, which is 13.5 



lo.O 



times as heavy as water. 



In order to obtain a more accurate mean pressure a large number of measure- 

 ments must be made, first, of the systolic pressure, and secondly, of the diastolic 

 pressure. The arithmetic mean between the averages of these pressures may be 

 taken to be the mean blood-pressure. More accurate determinations require piano- 

 metric measurements. 



4. Influence of Posture. Remove the clip from the artery, and 

 again record the blood-pressure. Raise the posterior extremities of the 

 animal vertically upward. Keep them in this position for ten seconds 

 and again lower them slowly. By this means a large quantity of blood 

 is forced into the head-circuit of the animal, occasioning a rise in the 

 carotid pressure. Such increases in blood-pressure are usually asso- 

 ciated with a reflex slowing of the heart. 



5. Compression of the Abdominal Aorta. The preceding result may 

 also be obtained by temporarily occluding important subdivisions of 

 the vascular system. Thus, if the thumb is pressed flat against the 

 spinal column somewhat below the left costal arch of the animal, the 

 resultant compresssion of the abdominal aorta must lead to an engorge- 

 ment of the head-circuit and a corresponding rise in the carotid pressure. 

 Observe that the sudden release of the compression usually causes the 

 pressure to fall below normal, and that several seconds then commonly 

 elapse before it again assumes its normal height. It is only natural 

 to assume that the sudden inrush of the blood into the previously empty 



