THE CIRCULATION OF THE BLOOD. 153 



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. 



b. As regards quality. The quality of the blood supplied to the heart 

 has a distinct eifect 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 cf the ves- 

 sels. 



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

 in the next Chapter. 



Circulation in the Capillaries. 



When the capillary circulation is examined in any transparent part 

 of a full grown living animal by means of the microscope (Fig. 139), the 

 blood is seen to flow with a constant equable motion ; the red blood-cor- 

 puscles moving along, mostly in single 

 file, and bending in various ways to ac- 

 commodate themselves to the tortuous 

 course of the capillary, but instantly re- 

 covering their normal outline on reach- 

 ing a wider vessel. 



It is in the capillaries that the chief 

 resistance 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 contact. Alien Thomson). 



At the circumference of the stream in the larger capillaries, but chiefly 

 in the small arteries and veins, in contact with the walls of the vessel, 

 and adhering to them, there is a layer of liquor sanguinis which appears 

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

 ferred both from the general fact that such a 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 witli comparative rapidity ; the colorless lymph- 

 corpuscles 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 be at rest ; for if any of the corpuscles happen to be forced within it, 



FIG. 139. -Capillaries ca in the web 



