THE BLOOD FLOW 409 



presents a clear outer zone, measuring about 0.01 mm. ia. width and 

 containing only plasma and a few leukocytes, as well as a dark central 

 zone in which the red cells are massed. The platelets occupy the 

 peripheral layers of the stream. This arrangement is also evident 

 in the venules, but as the venous current is less rapid, the red cells 

 are more widely scattered and the marginal zone is not so clearly 

 defined. In the capillaries, very naturally, the distribution of the 

 corpuscular elements cannot be dominated so much by ordinary 

 physical conditions, because these channels are so small that one or 

 two erythrocytes placed side by side fill them completely. Another 

 means of differentiating between the true capillaries and their supply 

 and collecting tubules is presented by the color of the blood. It is 

 darkest in the venules owing to the presence of greater amounts of car- 

 bon dioxid, and lightest in the capillaries, because the red cells are here 

 spread out in thin layers and single cells, as has been mentioned above, 

 are practically colorless. Still another means of differentiation is 

 furnished by the structural appearance of the different blood-vessels. 

 As the wall of a true capillary is composed of only a single row of 

 flattened cells, it cannot be made out very clearly. Neither is it 

 possible to focus a venule very sharply. The arterial capillaries, on 

 the other hand, are generally well defined. This is especially true of 

 the arterioles, owing to the deposition of smooth muscle cells within 

 their wall. Moreover, these tubules generally pursue a serpentine 

 course, whereas the venous tubules are rather straight. 



The Circulation Time. A droplet of blood leaving the left ven- 

 tricle may pursue many different courses. It may enter the coro- 

 nary circuit and return to its starting point within a very short time, 

 or it may pass through the portal organs, the posterior extremity, the 

 brain and other parts, in which cases a very much longer period of 

 time will be required before it can again reach the cardiac vestibule. 

 E. Hering 1 attempted to determine the time required to complete the 

 circuit of the vascular system by introducing a chemical substance 

 into the blood which could be easily recognized. He made use of 

 solutions of potassium ferrocyanid which were injected into the right 

 external jugular vein and were tested for in the blood withdrawn 

 from the corresponding vein on the opposite side. These samples were 

 arranged in series in accordance with the time of their withdrawal 

 and were permitted to clot, after which the serum was tested with 

 ferric chlorid. The results showed that the solution completed the 

 circuit through the heart and carotid arteries in from 20 to 30 seconds. 

 Vierordt 2 made use of a more accurate method for determining the 

 length of the intervening period by permitting a series of receiving 

 cups to rotate at a uniform speed below the vein. Hermann employed 

 sodium ferrocyanid and permitted the blood to drop at regular inter- 

 vals upon paper moistened with ferric chlorid. 



1 Zeitschr. fiir Physiol., iii, 1829. 



2 Erschein. und Gesetze der Stromgeschw. des Blutes, Frankfurt, 1858. 



