RELATIONS OF CAPILLARY CIRCULATION TO RESPIRATION 67 



In the true capillaries, the blood is distributed in every direction, 

 in vessels of nearly uniform diameter. The vessels usually are so 

 small as to admit but a single row of corpuscles. In a single vessel, 

 a line of corpuscles may be seen moving in one direction at one mo- 

 ment, and a few moments after, taking an opposite course. When the 

 circulation is normal, the movement in the capillaries is always quite 

 slow as compared with the current in the arterioles, and is continuous. 

 Here, at last, the intermittent impulse of the heart is lost. The 

 corpuscles do not necessarily circulate in all the capillaries that are 

 in the field of view. Certain vessels may not receive a corpuscle for 

 some time, but afterward, one or two corpuscles become engaged in 

 them and a current is established. In some of the vessels of smallest 

 size, the corpuscles are slightly deformed as they pass through. The 

 scene is changed with every different part that is examined. In the 

 tongue, in addition to the arterioles and venules and the rich network 

 of capillaries, dark-bordered nerve-fibres, striated muscular fibres, 

 and epithelium can be distinguished. In the lungs large polygonal 

 air-cells are observed, bounded by capillary vessels in which the cor- 

 puscles move with great rapidity. It has been observed, also, that the 

 larger vessels in the lungs are crowded to their utmost capacity with red 

 corpuscles. 



Pressure of Blood in the Capillaries. There is, apparently, no way 

 of directly estimating the pressure of blood in the capillaries. If, how- 

 ever, a glass plate is placed on a part in which the capillary circulation 

 is active and is weighted until the subjacent capillaries are emptied, an 

 approximate idea of the blood-pressure in the vessels may be obtained. 

 Experiments made in this way, by Von Kries, show that the pressure 

 in the capillaries of the hand raised above the head is equal to a little 

 less than one inch (24 millimeters) of mercury ; in the hand hanging 

 down, a little more than two inches (54 millimeters) ; and in the ear, 

 about 0.8 of an inch (20 millimeters.) 



Rapidity of the Capillary Circulation. The current in the capillaries 

 of a part is subject to such variations, and the differences in different 

 situations are so considerable, that it is impossible to give a definite rate 

 that will represent the general rapidity of the capillary circulation ; and 

 in view of the uncertainty of the methods employed, it seems unnecessary 

 to discuss this question fully. Volkmann calculated the rapidity in the 

 mesentery of the dog and found it to be 0.02 to 0.03 of an inch (0.5 to 

 0.75 millimeters) per second. 



Relations of the Capillary Circulation to Respiration. The immediate 

 effects of asphyxia on the circulation are referable to the general capil- 

 lary system. In a series of experiments made on frogs, in 1857 (Flint), 



