676 SPECIAL PHYSIOLOGY. 



adhesive than the red corpuscles, as is shown by their clinging to a 

 glass slide or cover, as seen when a minute drop of blood is spread out 

 and examined under the microscope. 



The actual rate of motion of the blood in the capillaries, as watched 

 and measured by aid of the micrometer in the field of the microscope, 

 in the case of individual blood particles, has been found by various 

 observers to be, in the frog's web, rather more than 1 inch per minute, 

 in other words, about g^th of an inch per second (Hales, Valentin, 

 Weber). As to the warm-blooded animals, the rate of motion is higher, 

 being, according to Volkmann, 1.8 inches per minute in the dog ; 

 whilst the observations of Ludwig and Vierordt on the entoptical 

 retinal image, or image of Purkinje, give a velocity in the retinal 

 capillaries of their own eyes, of from about l^th inch to rather more 

 than If inch per minute. The average velocity in Man might there- 

 fore fairly be estimated at about 2 inches per minute, or g^th of an 

 inch per second, i. e., twice the velocity in the frog. The apparent 

 rate of motion of the blood in the capillaries of either a warm or cold- 

 blooded animal, as seen under the microscope, is so high that the ob- 

 server is apt to be misled with regard to its actual velocity ; and, 

 deceived by the apparent motion, to doubt that the real velocity is 

 only 1 inch per minute in the cold-blooded animal, and 2 inches per 

 minute in the warm-blooded animal. But the area of observation 

 being enormously magnified, the apparent or angular motion of the 

 blood before the eye of the observer, is increased in the same propor- 

 tion ; so that in the field of a microscope magnifying 180 diameters, 

 the rate of motion of the capillary blood appears to be, in the frog, 

 180 inches per minute, or 3 inches per second, and in the warm-blooded 

 animal, to equal twice that velocity. The actual slow rate of motion 

 of the blood through the capillaries, is remarkable and important in 

 connection with the nutritive, secernent, and respiratory functions, 

 giving ample time, as it were, for the important interchanges between 

 the blood and the tissues, or the air, which take place in them, espe- 

 cially for those of deoxygenation and oxygenation, which occur, the 

 former in the systemic, the latter in the pulmonary capillaries. 



The slow rate of motion of the blood in the capillaries, is even more 

 striking, when it is compared with the rate of motion in the arteries, 

 which, as already mentioned, is estimated at about 10 inches per second, 

 or 600 inches per minute, in the human carotid, so that the velocity of 

 the blood in the systemic arteries, is 300 times greater than that in 

 the systemic capillaries. It has been calculated that in the pulmonary 

 capillaries, the rate of motion of the blood is five times greater than 

 the average rate in the systemic capillaries, i. e., 10 inches per minute, 

 or the |th of an inch per second. 



This remarkable retardation of the blood in the capillary vessels, as 

 compared with its velocity in the arteries, is doubtless in part due to 

 increased friction, dependent on the vast increase in the number of 

 channels through which the blood now has to pass ; but its chief cause 

 is the very great increase in the capacity of the capillary, as compared 

 with that of the arterial system. It has already been stated that the 

 combined sectional areas of the first and second degrees of arterial 



