202 MAIN FEATURES OF APPARATUS. [BOOK i. 



the only wall of a capillary is elastic, permitting the channel offered 

 by the same capillary to differ much in width at different times, 

 to widen when blood plasma and blood corpuscles are being pressed 

 through it and to narrow again when the pressure is lessened or 

 cut off. The same thin sheet permits water and substances, 

 including gases, in solution to pass through itself from the blood 

 to the tissue outside the capillary and from the tissue to the 

 blood, and thus carries on the interchange of material between the 

 blood and the tissue. In certain circumstances at all events white 

 and even red corpuscles may also pass through the wall to the 

 tissue outside. 



The minute arteries and veins with which the capillaries are 

 continuous allow of a similar interchange of material, the more so 

 the smaller they are. 



The walls of the veins are thinner, weaker and less elastic 

 than those of the arteries, and possess a very variable amount of 

 muscular tissue ; they collapse when the veins are empty. Though 

 all veins are more or less elastic and some veins are distinctly 

 muscular, the veins as a whole cannot, like the arteries, be 

 characterized as eminently elastic and contractile tubes; they 

 are rather to be regarded as simple channels for conveying the 

 blood from the capillaries to the heart, having just so much 

 elasticity as will enable them to accommodate themselves to the 

 quantity of blood passing through them, the same vein being at 

 one time full and distended and at another time empty and 

 shrunk, and only gifted with any great amount of muscular 

 contractility in special cases for special reasons. The united 

 sectional area of the veins, like that of the arteries, diminishes 

 from the capillaries to the heart; but the united sectional area 

 of the venaB cavae at their junction with the right auricle is 

 greater than, nearly twice as great as, that of the aorta at its 

 origin. The total capacity also of the veins is much greater than 

 that of the arteries. The veins alone can hold the total mass of 

 blood which in life is distributed over both arteries and veins. 

 Indeed nearly the whole blood is capable of being received by 

 what is merely a part of the venous system, viz. the vena portae 

 and its branches. 



