THE CIRCULATION OF THE BLOOD AND LYMPH iiy 



Harvev 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, it is easy to calculate the 

 velocity of the capillary blood-stream. It has been estimated 

 at from o - 2 to 0*8 mm. per second in different parts and different 

 animals. 



The comparative slowness of the current and the disappear- 

 ance of the pulse are the chief characteristics of the capillary 



FIG. 49. 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. 



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 ^ (5 to 20 p 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 it undergoes a great and sudden increase. At the venous 

 end of this region the cross-section is again somewhat abruptly 

 contracted, and then gradually lessens as the right side of the 



