n8 A MANUAL OF PHYSIOLOGY 



the presence of circularly-arranged muscular fibres in the 

 arterioles, their absence in the capillaries has its physio- 

 logical correlative. The calibre of the 'arterioles can be 

 altered by contraction of these fibres under nervous in- 

 fluences ; the calibre of the capillaries, although it varies 

 passively with the blood-pressure, and is possibly to some 

 extent affected by active contraction of the endothelial cells, 

 cannot be under the control of vaso-motor nerves acting on 

 muscular fibres. 



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 



Fro. 41. DIAGRAM TO ILLUSTRATE THE SLOPE OF PRESSURE ALONG 

 THK 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. 



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, it is easy to calculate the velocity of the 

 capillary blood-stream. It has been estimated at from 

 2 to *8 mm. per second in different parts and different 

 animals. 



The comparative slowness of the current and the dis- 



