THE PULSATORY VARIATIONS IN BLOOD PRESSURE 379 



the heart and gradually becomes less apparent in the direction of the 

 distal channels. In the capillaries, these pulse waves are usually not 

 in evidence, because the friction encountered in this particular division 

 of the vascular system is so great that the fluctuations in pressure are 

 completely neutralized. But, in the event of a capillary dilatation, 

 this resistance is usually diminished to such an extent that the individ- 

 ual pulsations are able to extend directly into the distalmost veins. 

 This phenomenon is often observed in glands during secretion, because 

 their activity necessitates a copious supply of blood and hence, an 

 injected state of their capillaries. In the submaxillary gland, this 

 vasodilatation may be produced by stimulation of the chorda tym- 

 pani nerve. The arterial pulse is then clearly visible in the small vein 

 draining this organ. 



FIG. 198. SPHYGMOGRAM FROM THE RADIAL ARTERY, DUDGEON SPHYGMOGRAPH. 

 D, the dicrotic wave;P, the predicrotic wave. (HoweU.) 



The Frequency of the Arterial Pulse. It is evident that the number 

 of the pulse-waves must coincide precisely with the frequency of the 

 heart, because the cardiac output forms the basis of these oscillations. 

 For this reason, the palpation of the pulse in such arteries as the radial, 

 brachial, temporal, or carotid, is practised primarily for the purpose of 

 ascertaining the cardiac frequency. As this topic has been dealt with 

 at length in a preceding chapter, it need not be discussed further at 

 this time. Attention should, however, be called to one or two points 

 of clinical value. 



Under certain abnormal conditions, it may happen that some of 

 the ventricular contractions do not develop a power sufficient to raise 

 the semilunar valve flaps, or, if they do, are quite unable to overcome 

 the general arterial pressure. In the first instance, the cardiac efforts 

 fail absolutely in producing pulse-waves, and in the second, in sus- 

 taining them for any considerable distance. This is generally true of 

 the so-called extrasystoles which, as the name indicates, are special 

 contractions interposed between the regular ones. As long as these 

 extra efforts of the ventricles do not interfere with the general rhythm 

 and output of the heart, no circulatory disturbances result. In further 

 illustration of this fact, that the frequency of the pulse does not always 

 indicate the rate of the heart, might be mentioned the condition of 

 heart-block, during which, as has been stated above, the auricular rate 

 is maintained, while the number of the ventricular contractions is 

 diminished. Thus, it may be gathered that the best policy is to bring 



