THE CIRCULATION OF THE BLOOD 



377 



friction between successive layers of the blood-stream, the resistance of the 

 blood flow, and the development of the blood-pressure. The relative 

 breadth of the still layer and amount of friction are greater in small than in 

 large vessels. 



The volume of blood passing into any given capillary area is determined 

 by the degree of contraction of the arterioles. Thus on the application of 

 warm saline solution, which relaxes the arterioles, there is a large increase 

 in the inflow of blood; vessels previously invisible suddenly come into view 

 as the blood with its corpuscles passes into them. On the application of 

 cold water, which contracts the arterioles and di- 

 minishes the inflow, many of the smaller vessels 

 entirely disappear from view. The contraction and 

 relaxation of the arterioles will therefore deter- 

 mine the quadtity of blood flowing into and 

 through the capillary system. 



Migration of the White Corpuscles. A 

 phenomenon frequently observed in the capillary 

 vessels of the mesentery or of the bladder of the 

 frog is the passage of the white corpuscles through 

 the walls into the surrounding lymph-spaces. To 

 this process the term migration or diapedesis is 

 given. After the tissues have been exposed to the 

 air for some time or subjected to an irritant, the 

 vessels dilate and become distended with blood. 

 In a short time the blood-stream slows, and finally 

 comes to rest. The condition of stasis is then 

 established. During the development of this condi- 

 tion the white corpuscles accumulate in large num- 

 bers along the inner surface of the vessels and soon 

 begin to pass through the vessel-walls. This they 

 do by protruding a portion of their substance and 

 inserting it into and through the vessel-wall. This 

 once accomplished, the remainder of the cell in due 

 time follows until it has entirely passed out into the 

 tissue-space. The opening in the vessel-wall now 

 closes. The successive steps in this process are 

 shown in Fig. 175. As this migration occurs 

 mainly after the circulation has ceased or when 

 the tissues present the phenomena of approaching 

 inflammation, it is difficult to state in how far it is strictly a physiologic 

 process. 



The Venous Circulation. The blood, having passed through the 

 capillary vessels, is gathered up by the veins and conveyed to the right side 

 of the heart. As the veins converge and unite to form larger and larger 

 trunks the sectional area gradually diminishes, and hence the velocity of the 

 blood-flow increases, though it never attains the velocity, even in the venae 

 cavae, that it had in the aorta, for the reason that the sectional area of the 

 venae cavae is considerably larger than that of the aorta. The pressure also 

 is very low in the larger veins because the friction still to be overcome is 

 relatively very slight. 



FIG. 175. DIAGRAM TO 

 SHOW VARIOUS STAGES IN 

 THE DIAPEDESIS OR Ml- 

 GRATION OF WHITE COR- 



PUSCLES. 



