158 HAND-BOOK OF" PHYSIOLOGY. 



stracted, the vaso motor apparatus must counteract the great increase of 

 pressure by dilating the small vessels, and so diminishing the peripheral 

 resistance, for after each rise there is a partial fall of pressure; and after 

 the limit is reached the whole of the injected blood displaces, as it were, 

 an equal quantity which passes into the small veins, and remains within 

 them. It should be remembered that the veins are capable of holding 

 the whole of the blood of the body. 



The amount of blood supplied to the heart both to its substance and 

 to its chambers, has a marked effect upon the blood-pressure. 



1). As regards quality. The quality of the blood supplied to the heart 

 has a distinct effect upon its contraction, as too watery or too little oxy- 

 genated blood must interfere with its action. Thus it appears that blood 

 containing certain substances affects the peripheral resistance by acting 

 upon the muscular fibres of the arterioles themselves or upon the local 

 centres, and so altering directly, as it were, the calibre of the vessels. 



5. Respiratory changes affecting the blood-pressure will be considered 

 in the next Chapter. 



CIRCULATION IN THE CAPILLARIES. 



When seen in any transparent part of a living adult animal by means 

 of the microscope (Fig. 140) the blood flows with a constant equable mo- 

 tion; the red blood-corpuscles moving along, mostly in single file, and 

 bending in various ways to accommodate themselves to the tortuous course 



of the capillary, but instantly recovering their 

 normal outline on reaching a wider vessel. 



It is in the capillaries that the chief resist- 

 ance is offered to the progress of the blood; 

 for in them the friction of the blood is greatly 

 increased by the enormous multiplication of 

 the surface with which it is brought in con- 

 tact. 



At the circumference of the stream in the 

 larger capillaries, but chiefly in the small arte- 

 FIG. 140. capillaries (o in the ries and veins, in contact with the walls of 



web of the frog s foot connecting a 



small artery (A) with a small vein v the vessel, and adhering to them, there is 



(after Allen Thomson). / . . 



a layer of liquor sangumis which appears to 



be motionless. The existence of this still layer, as it is termed, is 

 inferred both from the general fact that such an one exists in all fine 

 tubes traversed by fluid, and from what can be seen in watching the move- 

 ments of the blood-corpuscles. The red corpuscles occupy the middle of 

 the stream and move with comparative rapidity; the colorless lymph-cor- 

 puscles run much more slowly by the walls of the vessel; while next to 

 the wall there is often a transparent space in which the fluid appears to 



