viii BLOOD-STREAM: MOVEMENT IN VESSELS 263 



which can be measured by an ocular micrometer. The values 

 obtained with this method by different observers are shown in the 

 following table by Tigerstedt : 



The velocity of the red corpuscles in the capillaries of the retina 

 was ingeniously determined by Vierordt upon himself, by means 

 of their entoptic images. If we look without accommodating the 

 eye at a large clear surface, e.g. the sky, numerous shining points 

 appear which move one after the other by tortuous paths ; these 

 are the blood-corpuscles seen entoptically, owing perhaps to con- 

 centration of light upon their concave discs. Vierordt projected 

 these images upon a surface of 11-16 cm. from the eye, and there 

 determined the space traversed by a single corpuscle in the time- 

 unit. From this he deduced a velocity of 0'5-0'9 mm. per sec. 

 The values of the velocity of blood -flow in the capillaries is of 

 special interest, owing to the fact that it is possible from these to 

 deduce the approximate extent of the total area of the capillary 

 system. Since the velocity of the blood in the vascular system is 

 inversely proportional to its total area, we can calculate from the 

 area of the aorta and the velocity of the blood in the aorta and its 

 capillaries the total area of these last, inasmuch as the area of the 

 capillary system 



_ area of aorta x velocity in aorta 

 velocity in capillaries 



If we admit the value of O'5-l in. as the mean velocity in the 

 aorta and that of O'5-l mm. in the capillaries per second, thus, with 

 an aortic area of 4 '4 cm.' 2 , we obtain the value of 8800-2200 cm.' 2 for 

 the total area of the capillary system (Tigerstedt) 



VII. We must now pass on to the methods devised in order to 

 analyse the pulsatory oscillations of blood pressure, of current 

 velocity, and of the volume of the vessels. All three phenomena 

 are intimately related among themselves, and all depend on the 

 fact that the flow of blood from the heart to the arteries is not 

 continuous, but occurs in intermittent waves, which coincide with 

 the cardiac systole. We will begin by reviewing, collectively, the 



VOL. i s a 



