The blood, when in a fresh state, is apparently colored and opaque, but if a minute portion is examined with a microscope, it is seen to be a comparatively clear liquid in which float numerous reddish disk-like bodies known as corpuscles, also blood plates and blood granules. These bodies give to the blood its bright red color. The liquid in which they are suspended, a clear amber yellow liquid, is called the plasma.

131. Corpuscles

The corpuscles are not mere masses of unformed matter, but they are minute bodies having a definite form and structure. They make up from 35 to 40 per cent of the blood, and contain over 30 per cent of dry matter. This dry matter consists mostly of haemoglobin, a compound that is peculiar to the blood, and equips it for one of its most important offices.

132. Haemoglobin

Haemoglobin, as before stated, is made up of a protein (globin), and a coloring matter (haimatin), in the latter of which is combined a definite proportion of iron. The peculiar property of this compound, which renders it so useful a constituent of the blood, is its power of taking up oxygen and holding it in a loose combination until it is needed for use throughout the body. When thus charged, it is known as oxyhemoglobin. Because of this function of their most prominent constituent, blood corpuscles become the carriers of oxygen to all parts of the body. They are also concerned in gathering up one of the waste products of the nutritive changes, viz., carbon dioxid, which is conveyed by them in loose chemical combination to the point where it may be thrown off from the body. Hoematin may also unite with other compounds, as, for instance, carbon monoxid, which displaces and excludes oxygen and is disastrous in its effects.

Fig. 8. - Red and white corpuscles of blood (magnifies!). A, red corpuscles; a, a, white corpuscles; B. C, D. red corpuscles, much magnified; F, G, white corpuscles, much magnified.

133. Leucocytes

The blood also contains amceba-like bodies know as white corpuscles, that are variable in shape and constantly changing in form. These are sometimes called leucocytes, and are regarded as having an important function. They may increase with extraordinary rapidity, especially around centers of infection and inflammation, and it is regarded as proven that they endeavor to destroy foreign bodies in the blood and also render harmless the injurious products coming from the activity of micro-organisms. They evidently have other functions not well understood, for it is noticed that they accumulate in large numbers during intestinal digestion. Very likely they act as a means of transportation, and they probably play some part in metabolism in accomplishing certain exchanges of nutrient substances.

134. The Plasma

The plasma is about nine-tenths water, so that it easilv holds in solution whatever soluble nutrients are discharged into it from the alimentary canal.

Among its constituents are found members of all the classes of compounds that are important in this connection, - ash, protein, carbohydrates, and fats. The proportion of ash is about 1 per cent, three-fourths of it being common salt, and the remainder consisting of phosphoric acid, lime, and other important mineral compounds. The solid matter of the plasma is rich in proteins, including the fibrinogen, which is the mother substance of fibrin, and several albumins and globulins. These proteids make up about 80 per cent of the total dry substance of the plasma. Sugar and fats are also present, their proportions varying somewhat with the extent to which they are being absorbed from the digestion of food. In fact, the blood carries not only its characteristic and permanent constituents, but also the nutrients absorbed from the alimentary canal. It is evident that the blood is charged with those materials which we recognize as necessary to the construction and maintenance of the animal body. The plasma also constantly contains very small proportions of the end products of metabolism, such as urea and uric acid and waste bile products which are being transported to the points of excretion. It also holds in solution, or as carbonates, some of the carbon dioxid gathered up in the circulation of the blood through the tissues.