928 PHYSIOLOGY 



but he would regard the more pronounced dicrotism of the pulse, of which examples 

 have been given earlier, as due for the most part to the reflection of waves from the 

 periphery. 



Prom time immemorial the physician has sought by feeling the pulse 

 to come to some idea as to the condition of the circulation. A number of 

 different qualities have therefore been distinguished. According to the 

 number of beats per minute the pulse is distinguished as frequent or rare. 

 The size of the pulse has reference to the amplitude of excursions of each 

 beat and the pulse is distinguished as large or small. The velocity of the 

 pulse expresses the speed with which the excursion is accomplished. The 

 quick pulse is one in which the artery presses against the finger suddenly 

 and then disappears suddenly, while in the slow pulse the period during 

 which pressure can be felt is more prolonged. The hardness of the pulse 

 is determined chiefly by the blood-pressure. If the pulse is compressible 

 it is spoken of as soft; if it can only be obliterated with difficulty it is 

 hard. Certain combinations of these qualities are also described. Thus 

 a large and hard pulse is spoken of as strong, a weak pulse being both small 

 and soft. A small hard pulse is called contracted. If the rhythm of the 

 heart-beat is irregular the pulse is also irregular. An intermittent pulse is 

 one in which one heart-beat is dropped occasionally, i.e. once in every 

 four or eight beats, and may be due to the interposition of a ventricular 

 contraction which is too weak to send the pulse along so far as the radial 

 artery. 



Judgments as to the conditions of the heart and circulation from the 

 feeling of the pulse oscillations must, however, be made with extreme 

 caution. The pulse-curve may, indeed, give approximate information as 

 to the condition of things in the heart. Thus the period between the 

 beginning of the primary elevation and the dicrotic notch corresponds to 

 the outflow of blood from ventricle to aorta. A large pulse-curve does not 

 necessarily indicate a big output, since the expansion of the artery is 

 determined not only by events occurring in the aorta but also by the tonus 

 of the artery under the finger and the resistance in the peripheral branches. 

 Perhaps the best-marked condition of the pulse is that known as the 

 * water-hammer ' pulse, which is observed in cases where the aortic valves 

 are injured or diseased so as to allow of regurgitation into the ventricle. 

 The systolic rise of pressure in the arterial system is followed by an extremely 

 rapid fall, so that towards the end of diastole the pressure in the arteries 

 may be insufficient to keep the arterial system filled. Under such con- 

 ditions, if the arm be held above the head and the wrist of the patient 

 be grasped, the pulse in the arteries of the wrist is felt as a smart blow 

 coinciding with each beat of the heart. 



THE CIRCULATION THROUGH THE CAPILLARIES 

 The capillary circulation is most easily studied by examining under 

 the microscope the tongue of the frog or the web of the frog's foot. Under 

 a power of about 150 to 180 diameters a network of vessels is seen, 



