324 TEXT-BOOK OF PHYSIOLOGY. 



out into the tissue-space. The opening in the cell- wall now closes. 

 The successive steps in this process are shown in Fig. 155. 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 sec- 

 tional 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^Hor the reason that the sectional area of 

 the venae cavae is considerably larger than that of the aortaN^The 

 pressure also is very low in the larger veins because the friction still 

 to be overcome is relatively very slight. 



\vThe capacity of the venous system is considerably greater than 

 that of the arterial system, as there are usually two and even three 

 veins accompanying each artery X This, taken in connection with its 

 greater distensibility, makes of the venous system a reservoir in which 

 blood can be stored. On this reservoir the arterial system can call 

 for that amount of blood necessary for the maintenance of its normal 

 volume and pressure, and into it any excess can be discharged. 

 The relative amounts contained in the two systems are regulated by 

 the nervous system. The movement of the blood through the veins 

 is accomplished by the cooperation of several forces, reference to 

 which will be made in a following paragraph. 



THE PULMONIC VASCULAR APPARATUS. 



The pulmonic vascular apparatus consists of a closed system 

 of vessels extending from the right ventricle to the left auricle, and 

 includes the pulmonary artery, capillaries, and pulmonary veins. In 

 its anatomic structure and physiologic properties it closely resembles, 

 it it is not identical with, the systemic apparatus. 



The stream-bed widens from the beginning of the pulmonary 

 artery to the middle of the capillary system; it again narrows from 

 this point to the terminations of the pulmonary veins. 



The movement of the blood from the beginning to the end of the 

 system is due to a difference of pressure between these two points, 

 the result of the friction between the blood and the vascular walls, 

 brom the difference in the extent of the pulmonic and systemic 

 systems it is evident that, other things being equal, the friction is less, 

 and therefore also the pressure is less in the former than in the latter 



