178.  Physical Properties of Blood.  The blood has been called the life of the body from the fact that upon it depends our bodily existence.  The blood is so essentially the nutrient element that it is called sometimes very aptly “liquid flesh.”  It is a red, warm, heavy, alkaline fluid, slightly salt in taste, and has a somewhat fetid odor.  Its color varies from bright red in the arteries and when exposed to the air, to various tints from dark purple to red in the veins.  The color of the blood is due to the coloring constituent of the red corpuscles, haemoglobin, which is brighter or darker as it contains more or less oxygen.

[Illustration:  Fig. 65.—­Blood Corpuscles of Various Animals. (Magnified to the same scale.)

  A, from proteus, a kind of newt;
  B, salamander;
  C, frog;
  D, frog after addition of acetic acid, showing the central nucleus;
  E, bird;
  F, camel;
  G, fish;
  H, crab or other invertebrate animal
]

The temperature of the blood varies slightly in different parts of the circulation.  Its average heat near the surface is in health about the same, viz. 98-1/2 degrees F. Blood is alkaline, but outside of the body it soon becomes neutral, then acid.  The chloride of sodium, or common salt, which the blood contains, gives it a salty taste.  In a hemorrhage from the lungs, the sufferer is quick to notice in the mouth the warm and saltish taste.  The total amount of the blood in the body was formerly greatly overestimated.  It is about 1/13 of the total weight of the body, and in a person weighing 156 pounds would amount to about 12 pounds.

179.  Blood Corpuscles.  If we put a drop of blood upon a glass slide, and place upon it a cover of thin glass, we can flatten it out until the color almost disappears.  If we examine this thin film with a microscope, we see that the blood is not altogether fluid.  We find that the liquid part, or plasma, is of a light straw color, and has floating in it a multitude of very minute bodies, called corpuscles.  These are of two kinds, the red and the colorless.  The former are much more numerous, and have been compared somewhat fancifully to countless myriads of tiny fishes in a swiftly flowing stream.

180.  Red Corpuscles.  The red corpuscles are circular disks about 1/3200 of an inch in diameter, and double concave in shape.  They tend to adhere in long rolls like piles of coins.  They are soft, flexible, and elastic, readily squeezing through openings and passages narrower than their own diameter, then at once resuming their own shape.

The red corpuscles are so very small, that rather more than ten millions of them will lie on a surface one inch square.  Their number is so enormous that, if all the red corpuscles in a healthy person could be arranged in a continuous line, it is estimated that they would reach four times around the earth!  The principal constituent of these corpuscles, next to water, and that which gives them color is haemoglobin, a compound containing iron.  As all the tissues are constantly absorbing oxygen, and giving off carbon dioxid, a very important office of the red corpuscles is to carry oxygen to all parts of the body.