130 THE CIRCULATION OF THE BLOOD AND LYMPH 



physiological correlative. The calibre of the arterioles can be 

 altered by contraction of these fibres under nervous influences ; the 

 calibre of the capillaries, although it varies passively with the blood- 

 pressure, and is possibly to some extent affected by active con- 

 traction of the endothelial cells, cannot be under the control of vaso- 

 motor nerves acting on muscular fibres (but see p. 171). 



Harvey had deduced from his observations the existence of 

 channels between the arteries and the veins. Malpighi was the first 

 to observe the capillary blood-stream with the microscope, and thus 

 to give ocular demonstration of the truth of Harvey's brilliant 

 reasoning. He used the lungs, mesentery and bladder of the frog. 

 The web of the frog, the tail of the tadpole, the wing of the bat, the 

 mesentery of the rabbit and rat, and other transparent parts, have 

 also been frequently employed for such investigations. From the 

 apparent velocity of the corpuscles and the degree of magnification, 



Fig 58. Diagram to illustrate the Slope of Pressure along the Vascular System. 

 A, arterial; C, capillary; V, venous tract. The interrupted line represents the 

 line of mean pressure in the arteries, the wavy line indicating that the pressure 

 varies with each heart-beat. The line passes below the abscissa axis (line of 

 zero or atmospheric pressure) in the veins, indicating that at the end of the venous 

 system the pressure becomes negative. 



it is easy to calculate the velocity of the capillary blood- stream. 

 It has been estimated at from 0-2 to 0-8 mm. per second in different 

 parts and different animals. 



The comparative slowness of the current and the disappearance 

 of the pulse are the chief characteristics of the capillary circulation. 

 The explanation we have already found in the great resistance of 

 the narrow arterioles and the much-branched capillary vessels. 

 Although the average diameter of a capillary is only about 10 p 

 (5 to 20 [JL in different parts of the body), the number of branches 

 is so prodigious that the total cross-section of the systemic capillary 

 tract has been estimated at 500 to 700 times that of the aorta. 

 Such estimates are, of course, by no means exact. 



The total cross-section of the vascular channel gradually widens 

 as it passes away from the left ventricle. In the capillary region 



