VELOCITY AND PRESSURE OF BLOOD-FLOW. 



461 



will then record mean pressure and show no variations with the 

 heart beat. A general idea of the variations in systolic, diastolic, 

 and mean pressures, throughout the arterial system, may be ob- 

 tained from the schema given in Fig. 184. 



Method of Measuring Systolic and Diastolic Pressure in 

 Animals. In animals a manometer may be connected directly 

 with the artery and systolic and diastolic pressures may be obtained 

 in one of two general ways: (1) By using some form of pressure 

 recorder or manometer sufficiently mobile to follow very quick 

 changes of pressure. (2) By using a mercuiy manometer pro- 

 vided with maximum and minimum valves. Of the manometers 

 that have been divised to register accurately the quick changes in 

 pressure due to the heart beat, the two that are most frequently 



Fig. 185. The spring manometer of Fick (after Langendorff) : f, The flat metal 

 tube filled with liquid; r, the lead tube connecting with the artery; h, h, h, the lever 

 mechanism of light wood communicating the movements of / to the writing point, s; p, a 

 small disc immersed in a vessel of oil to still further dampen the inertia swings. 



referred to in physiological literature are the spring manometer of 

 Tick* and the membrane manometer of Hiirthle.t 



The Fick manometer is shown in Fig. 185. It consists of a flat, hollow 

 metallic spring bent into the form of a C. The interior is filled with liquid 

 and is connected by rigid tubing also filled with liquid, with the interior of 

 the artery. The variations of pressure in the artery are transmitted to the 

 interior of the spring and tend to straighten it, thus causing corresponding 

 movements of the free end. Before or after using this instrument it mnst be 

 calibrated, that is, the variations in movement must be given absolute values 

 in terms of millimeters of mercury by ascertaining directly the extent of move- 

 ment caused by known pressures. The Hurthle manometer is more frequently 

 used at present. The principle made use of in this instrument is illustrated 

 by the diagram in Fig. 186. The instrument consists essentially of a small 

 box or tambour of very limited capacity; the top of the tambour is covered 



* Fick, "Archiv f. Physiologic/' 1864, p. 583. 



t "Archiv f. d. gesammte Physiologic," 49, 45, 1891. 



