vni BLOOD-STKEAM : MOVEMENT IN VESSELS 243 



convenience, however, it is customary to join the artery to the 

 manometer by a simple cannula, ligatured to the central side of 

 the artery, so that the vessel is occluded. In this case the height of 

 the manometer column of course expresses the lateral pressure in 

 the arterial trunk from which the occluded vessel sprang. The 

 cannula may also be introduced in a peripheral, instead of central, 

 direction in the artery. In this case the manometer measures the 

 pressure either in the capillary network or in the other arterial 

 branches with which the artery, to which the cannula is peri- 

 pherally connected, anastomoses. For instance, a manometer 

 connected by a cannula with the central side of the dog's carotid 

 measures the lateral pressure in the aoria ; if, on the other hand, 

 the cannula is connected with the peripheral side of the same 

 artery, the pressure that obtains in the so-called Circle of Willis 

 will be obtained. In the first case, the pressure, according to 



FIG. 92. Tracing of arterial pressure in dog's carotid, recorded by Lud wig's kymograph. 



(Marey.) 



Steiner, may reach a value of 214 mm. Hg ; in the second, a 

 pressure of only 154 mm. Hg results. The first value represents 

 the lateral pressure that prevails in the trunk of the aorta at the 

 origin of the carotid ; the second, the pressure in the arterial Circle 

 of Willis, which communicates directly with the peripheral trunk 

 of the carotid. 



As can be seen in Fig. 92, the tracings recorded with Lud wig's 

 manometer exhibit small oscillations corresponding with the single 

 cardiac systoles, and more ample and less steep oscillations which 

 correspond to the respiratory movements. 



The latter will be considered in relation to the mechanics of 

 respiration ; as regards the pulsatory oscillations it must be noted 

 that the mercury, owing to inertia, is incapable of faithfully 

 recording the rapid variations of arterial pressure produced by 

 each wave of blood expelled from .the heart, so that Ludwig's 

 instrument is the least fitted for the study of the form of the 

 pulse-wave. Since this instrument is intended to determine the 

 value of the average blood pressure, the pulsatory oscillations are 

 a superfluous complication which can easily be excluded by a 

 constriction in the manometer at one point, as proposed by 



