[Sidenote: 23. Difference in the appearance and composition of the blood? Temperature of the blood? The blood while passing through the lungs? The consequence?] 23. DIFFERENCE BETWEEN ARTERIAL AND VENOUS BLOOD.--The following table presents the essential points of difference in the appearance and composition of the blood, before and after its passage through the lungs:-- {135} _Venous Blood._ _Arterial Blood._ Color, Dark blue, Scarlet. Oxygen, 8 per cent., 18 per cent. Carbonic Acid, 15 to 20 per cent., 6 per cent., or less. Water, More, Less. The temperature of the blood varies considerably; but the arterial stream is generally warmer than the venous. The blood imparts heat to the air while passing through the lungs, and consequently the contents of the right side of the heart has a higher temperature than the contents on the left side. [Sidenote: 24. What do we learn by means of the spectroscope? "Carriers of oxygen?" Blue blood in the system?] 24. By means of the spectroscope, we learn that the change of color in the blood has its seat in the corpuscles; and that, according as they retain oxygen, or release it, they present the spectrum of arterial or venous blood. There evidently exists, on the part of these little bodies, an affinity for this gas, and hence they have been called "carriers of oxygen." It was long ago thought that blue blood was a trait peculiar to persons of princely and royal descent, and boastful allusions to the "_sang azure_" of kings and nobles are quite often met with. Physiology, however, informs us that blue blood flows in the veins of the low as well as the high, and that so far from its presence indicating a mark of purity, it, in reality, represents the waste and decay of the system. [Sidenote: 25. The amount of air that passes in and out of the lungs?] 25. AMOUNT OF RESPIRATORY LABOR.--During ordinary calm respiration, we breathe eighteen times in a minute; and twenty cubic inches of air pass in and out of the lungs with every breath. This is equivalent to the use of three hundred and sixty cubic inches, or more than ten pints of air each minute. From this we calculate that the quantity of air which hourly traverses the lungs is about thirteen cubic feet, or seventy-eight gallons; and daily, not {136} less than three hundred cubic feet, an amount nearly equal to the contents of sixty barrels. [Sidenote: 26. Air absorbed in its transit through the lungs? The loss? Carbonic acid exhaled? Effect of excitement or exertion? What estimate?]