408 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



in which a stands for the distance between the screen and the anterior 

 nodal point, b for the distance between the retina and the posterior 

 nodal point, and c for the distance traversed by the projected image. 

 Values between 0.6 and 0.9 mm. in a second have been found by this 

 method. If it is now remembered that the length of the true capil- 

 laries varies between 0.4 and 0.7 mm., the general conclusion may be 

 drawn that a red cell traverses a capillary of average length in about 

 1 second. 



The Circulation Observed under the Microscope. The study of 

 the blood flow was made possible at an early date by the discovery of 

 the microscope. To begin with, cold-blooded animals were employed, 

 partly because their tissues are more accessible and resistant, and 

 partly because their erythrocytes are much larger than those found 

 in warm-blooded animals. These observations may be arranged in the 

 following chronological order : 



Malpighi (1686) : Lung, mesentery, urinary bladder of the frog. 

 Leeuwenhoek (1689): Tail of the tadpole and fish, wing of the bat. 

 Cowper (1704): Mesentery of the rabbit. 

 Spallanzani (1773): Embryo of the chick. 

 Hueter (1879) : Mucous membrane of man. 

 Ewald (1896): Lung of the triton. 



When a capillary area is subjected to a magnification of about 15 

 diameters, it will be seen that many of its tubules are extremely 

 small and do not permit the passage of anything more than the plasma 

 and occasional white cells. Others, again, possess a somewhat larger 

 caliber and allow two or three red cells placed side by side to traverse 

 them. The most interesting picture, however, is presented in those 

 tubules which are just sufficiently large to permit the entrance of 

 single erythrocytes, so that it becomes possible to follow them as they 

 wend their way in single file through these circuitous passages. In 

 fact, in many cases these elements must be considerably elongated 

 before they can enter these tubules. They may be thrown across a 

 bifurcation and be rocked back and forth for some moments before they 

 manage to escape into one or the other of these branches. The latter 

 phenomenon, in particular, permits us to obtain a clear idea regarding 

 the elastic properties of these elements, as well as regarding the friction 

 and resistance which they must overcome in their journey through 

 these tubules. 



In general, it may be said that the principal characteristics of the 

 capillary flow are its slowness and constancy. The arterial capillaries 

 and arterioles are much larger than the capillaries proper and are, 

 therefore, able to accommodate a much greater number of red cells. 

 Furthermore, as the speed of flow within them is much greater, it is 

 difficult to distinguish the individual cells. The venous capillaries 

 and venules show essentially the same characteristics, but as the flow 

 within them is not so rapid, the different red cells may be more easily 

 differentiated from one another. On the arterial side, the stream 



