84 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



oscillate, conie to a standstill, and then reverse the direction of their move- 

 ment, and return to the capillary whence they had started. Naturally, no 

 such reversal will ever be seen in a capillary which springs directly from an 

 artery or which directly joins a vein. It will be remembered, however, that 

 any apparent speed of a corpuscle is much magnified by the microscope, aud 

 thai therefore the variations referred to are comparatively unimportant. We 

 may, in fact, without material error, treat the speed of the blood in the capil- 

 laries which intervene between the arteries and veins of a region as approxi- 

 mately uniform for an ordinary period of observation, as the minute varia- 

 tions will tend to compensate for one another. This speed is sluggish, as 

 alreadv noted. In the capillaries of the web of the frog's foot it has beeu 

 found to be about 0.5 millimeter per second. The causes of this sluggishness 

 will be set forth later. That the very short distance between artery and vein 

 is traversed slowly, deserves to be insisted on, as thus time is afforded for the 

 uses of the blood to be fulfilled. 



Capillary Blood-pressure. — The pressure of the blood against the capil- 

 lary wall is low, though higher than that of the lymph without. This pres- 

 sure is subject to changes, and is readily yielded to by the elastic and deli- 

 cate wall. From these changes of pressure changes of calibre result. The 

 microscope tells us less about the capillary blood-pressure than about the other 

 phenomena of the flow; but the microscope may sometimes show one striking 

 fact. In a capillary district under observation, a capillary not noted before 

 may suddenly start into view as if newly formed under the eye. This is 

 because its calibre has been too small for red corpuscles and leucocytes to enter, 

 until some slight increase of pressure has dilated the transparent tube, hitherto 

 filled with transparent plasma only. This dilatation has admitted corpuscles, 

 and has caused the vessel to appear. 



That the capillary pressure is low is shown, moreover, by the fact that when 

 one's linger is pricked or slightly cut, the blood simply drips away ; that it 

 does not spring in a jet, as when an artery of any size has been divided. That 

 the capillary pressure is low may also be shown, and more accurately, by the 

 careful scientific application of a familiar fact: If one press with a blunt 

 lead-pencil upon the shin between the base of a finger-nail and the neigh- 

 boring joint, the ruddy surface becomes pale, because the blood is expelled 

 from the capillaries and they are flattened. If delicate weights be used, 

 instead of the pencil, the force can be measured which just suffices to whiten 

 the surface somewhat, that is, to counterbalance the pressure of the distend- 

 ing blood, which pressure thus can be measured approximately. It has been 

 found to be very much lower than the pressure in the large arteries, con- 

 siderably higher than that in the large veins, and thus intermediate between 

 the two; whereas the blood-speed in the capillaries is less than the speed 

 in either the arteries or the veins. The pressure in the capillaries, meas- 

 ured by the method just described, has been found to be equal to that 

 required to sustain against gravity a column of mercury from 24 to 54 milli- 



