538 PHYSIOLOGY OP THE DOMESTIC ANIMALS. 



organs is not due to rhythmic contractions of the walls of the capillaries 

 but to the paralysis of the walls of the minute arterioles and the conse- 

 quent conduction of the impulse from the heart. 



The circulation of the blood through the capillaries admits of ready 

 study in the transparent tissues of different organs, such as the web of a 

 frog's foot, the lung of the frog, or the mesentery of the guinea-pio-. 

 When such tissue is subjected to microscopic examination in an animal 

 rendered motionless by the injection of curare, the blood may be seen 

 passing in a continuous stream from the smaller arterioles through the 

 capillaries to the veins (Fig. 225). The arterioles are readily recognized 

 by the greater velocity of the flow within them and by the fact that 

 occasionally faint pulsations synchronous with the contractions of the 

 heart may be recognized. Under ordinary circumstances when the 

 circulation through the capillaries is examined it will be found that the 

 corpuscles pass in single file with a velocity usually of about 0.51 mm. 

 a second. The calibre of the capillaries is, however, the seat of frequent 

 changes whose mechanism will be subsequently studied. Often we shall 

 find that the capillaries dilate, and we- have then a stream of corpuscles 

 moving several abreast through them, and even while undergoing 

 inspection the capillaries may be seen to become smaller in diameter 

 and occasionally in certain places so narrow as to refuse the passage 

 of a single corpuscle;- the blood then becomes blocked up behind this 

 contraction, and then we have channels dilating behind this obstruction 

 and carrying off the stagnated blood. 



The mean blood pressure in the capillaries has been placed at about 

 thirty-five millimeters of mercury, but it is evident that this pressure 

 must be subject to very great variations. 



When a microscopic examination is made of a frog's foot it is seen 

 that the file of nucleated corpuscles move with their axes parallel with 

 the stream, rotating sometimes on their axes, and occasionally we find 

 an evidence of the flexibility of the red blood-cells by noticing that 

 sometimes one of these cells, striking the bifurcation of a capillary, 

 will become doubled on itself, part lying in one branch and part in 

 another, until finally driven along by the cells coming behind it. 



The white blood-cells will be found to be moving with a much lower 

 velocity than the red blood-corpuscles (one-tenth or one-twelfth as fast), 

 rolling slowly along in contact with the walls of the capillaries outside 

 of the central, rapidly moving blood-current. Such a layer is termed the 

 inert layer, and in nearly all cases it will be found that, while the red 

 blood-cells move in a rapid stream through the centre of the vessel, a 

 clear space between this central column and the walls of the capillaries 

 may be recognized, in which inert layer, as already mentioned, the white 

 cells may be nearly always found. The presence of the white blood-cells 



