THE CIRCULATION. 305 



when the organ was excited to functional activity, it discharged the 

 same quantity in fifteen seconds. Thus the volume of blood passing 

 through the gland in a given time was more than four times as great 

 while in active secretion as in a condition of repose. 



The increased flow of blood, in a secreting gland, is accompanied by 

 an important change in its appearance. During repose, the blood, 

 which enters the submaxillary gland bright red, is changed from 

 arterial to venous, and passes out by the veins of a dark color. But 

 during active secretion, the blood is not only discharged in larger 

 quantity, but passes out by the veins of a red color, hardly distin- 

 guishable from that of arterial blood. When the secretion of the 

 gland is suspended, its venous blood again becomes dark-colored as 

 before. There is little doubt that the same is true of other glands, and 

 that the blood circulating in their capillaries is changed from red to 

 blue only during the period of functional repose ; while at the time of 

 active secretion it passes through the vessels in greater abundance, and 

 retains its ruddy color in the veins. 



This variation depends on the different functions performed by the 

 blood in the two periods. During glandular repose it serves for the 

 usual changes of nutrition, which consume its oxygen, and conse- 

 quently change its color from red to blue. But during active secretion 

 the blood passes in larger quantity, while its watery and saline ingre- 

 dients exude into the secretory ducts, bringing with them the materials 

 accumulated in the intervals of repose ; and as there is nothing in this 

 process to exhaust the oxygen of the blood, it therefore passes out by 

 the veins with its color comparatively unaltered. 



A similar ruddy color of the blood is to be seen in the renal veins, 

 where it is often nearly identical with that of arterial blood. "When 

 the kidneys are in a state of functional activity, the difference in color 

 between the renal veins and those of the neighboring muscles, or the 

 vena cava, is very marked. The only important change in the blood 

 while passing through the kidneys is the elimination of its urea ; the 

 process of local nutrition being altogether secondary. Consequently 

 the blood loses but little oxygen in these organs, and suffers but little 

 alteration of its hue. 



On the other hand, the venous blood coming from the muscles is 

 very dark, especially if they be in a state of active contraction ; and 

 as the muscles form so large a part of the mass of the body, their con- 

 dition has a preponderating influence on the color of the venous blood 

 in general. The greater the activity of the muscular system, the 

 darker is the blood returning from the trunk and extremities. In a 

 state of repose or paralysis, on the contrary, the change is less marked ; 

 and in the complete relaxation produced by abundant hemorrhage or 

 profound etherization, the blood in the larger veins often approximates 

 in color to that in the arteries. 



Finally, in the lungs the reverse process takes place. In these organs 

 the blood is supplied with a fresh quantity of oxygen, to replace that 



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