integrated very shortly after they are removed from the circulation. Although the number of red discs normally alters but little, on account of the constant changes occurring in the number of the white cells, the proportion of white to red varies much. It has been found to differ according to the observer, the situation, and other circumstances, as shown in the following table, which gives the number of red corpuscles to one colorless cell :

Observer's estimate of normal proportion :

Welcker,
Moleschott,

In various parts of the circulation :

Splenic vein,

Splenic artery,

Hepatic vein,
Portal vein,

According to age or sex :

Girls,

Boys,

Adult,

Old age,

Red. White.

In a disease of the spleen and lymphatic glands called Leucocythemia there may appear to be nearly as many white cells as red discs. Here, however, the red discs are deficient, while the colorless cells are multiplied.

THE COLORLESS CORPUSCLES.

The colorless cells of the blood, commonly called the white corpuscles, differ in no essential respect from the pale round cells which are found in most of the tissues of the body. They exist in large numbers in that fluid, namely, the lymph, which is drained back from the tissues into the blood, and occupy a great part of the lymphatic glands and spleen. They are often spoken of as lymphoid cells, leucocytes, indifferent, or formative cells, on account of their being so widely distributed throughout the tissues.

When fresh blood is examined with the microscope these cells can generally be seen adhering to the glass slide or cover glass and lying singly, apart from the groups of red discs. They can be recognized by their absence of marked color, finely granular structure, spherical shape, and the nuclei which may often be recognized near the centre of the cell. Though not always visible in fresh preparations, the nuclei can be brought to light by the action of many reagents-e. g., acetic acid. If examined while being moved by the blood current to the capillary vessels, they are seen to pass slowly along in contact with the vessel wall, while the red corpuscles rush rapidly past them down the centre of the channel (Fig. 98). This may partly be due to their peculiar adhesiveness, which also causes them to stick to the glass slide, while the red discs are washed away when a gentle stream of saline solution is allowed to flow under the cover glass. These cells show all the manifestations of activity characteristic of independent living beings. If kept in a medium suitable to them, and at the temperature of the body, they will soon be seen

to alter their appearance; their outline becomes faint, they are no longer spherical, but very irregular in shape, and constantly change their form by send

ing out and retracting pro-
cesses, by means of which
they change their position,
so that they may be said
to perform locomotion.
These movements are ren-
dered more active by a
slight increase of tempera-
ture, and are checked by
cold. For continued ob-
servation, about 38° C. is
the best temperature for
mammalian blood.
blood of the frog is gener-
ally used to see the motion
of the white corpuscles, as
warming is unnecessary in
the case of cold-blooded
animals. They respond to
many other influences,
such as electricity, etc.,

The

No doubt they continually absorb fluid nutriment from the surrounding medium, as is shown by the effect of poisons on them; and, by the repeated contractions and relaxations of parts of their substance in the form of pseudopodia, they appear to take into the inner parts of the protoplasm solid particles, which after some time are ejected after the manner of the small unicellular animals known as amoeba (p. 91).

While in motion in the circulation none of these amoeboid movements appear to take place, but when an arrest of the flow of blood in the capillaries occurs, they not only change their form, but also their position; and if there be no onward flow of blood for some little time, they creep out of the capillaries, pass

ing through the delicate vessel walls. This emigration of the blood cells is possibly a common event when a tissue is in need of textural repair. When excessive, it forms one of the most striking items of the series of events occurring in inflammation.

These cells differ much in size; generally they are somewhat larger than the red discs. Nothing like a cell wall can be seen to surround them, and from the movements above described it would appear certain that they are free masses of active protoplasm.

The number of white cells that can be collected is too small to allow of accurate chemical analysis, but there is no reason to suppose that they differ from other forms of protoplasm.

ORIGIN OF THE COLORLESS BLOOD CELLS.

Since such an ordinary circumstance as a hearty meal can materially influence the numbers of the white corpuscles, it would appear that they must be usually undergoing rapid variations in their number-probably by their being constantly used up and periodically replaced by new ones. The places in which they occur in greatest number are the lymphatic glands, the spleen, and the lymph follicular tissue in the intestinal tract.

There is no doubt that the lymph contains a much larger proportion of these cells after it has passed through the lymph glands, and the blood coming from the spleen contains an excessive proportion of them.

It is then not unreasonable to suppose that many of the white cells found in the blood have their origin in these organs.

They may also be developed from similar cells in any tissue, but their reproduction by division, other than that which probably occurs in the lymph follicles where it cannot be seen, is a circumstance of the greatest rarity, and few observers have been fortunate enough to witness the phenomenon.

The destiny of the white blood cells is probably manifold. From the readiness with which they escape from the capillaries and wander by their amoeboid movement through the neighboring tissues to reach any point of injury, it would appear that they take an active part in the repair of a tissue whose vitality has in

any way suffered. During the growth of all tissues these cells seem to contribute active agents to their formation; thus in the formation of bone it has been stated that escaped blood cells or their immediate offspring help to lay down the calcareous material, and some even settle themselves as permanent inhabitants of the lacunæ.

Further, they are in all probability the means of renewing the red discs. Their protoplasm either takes up the coloring matter from its surroundings, or forms it within itself from suitable ingredients. Certain it is that cells are found which are recognizable as white blood cells, which have more or less of the red coloring matter imbedded in their substance. As this increases, the cell gradually loses its distinctive characters and assumes those of a red corpuscle. Such elements, it will be seen, are common in the spleen and the blood leading from it.

THE COLORED CORPUSCLES.

The red discs were discovered in the human blood by Leuwenhoek, about 1673. They give the red color which characterizes the blood of all vertebrated animals (except the amphioxus), but are not found in the blood of the invertebrata, which only contains colorless cells. When the blood of the invertebrates has a color it owes it to the fluid, not to the corpuscles. The individual discs when viewed singly under the microscope appear to be pale orange, but when in masses the red becomes apparent.

The shape of the corpuscles differs in different classes of animals. In man and all other mammalia they are discs, concave on each side and rounded off at the margin. The only class of mammals which forms an exception to this rule is the camelidæ, whose red corpuscles are elliptical in shape, like those of non-mammalian vertebrates.

The corpuscles of birds, amphibia and fish are flattened, elliptical plates, slightly convex on each side, and containing a distinct oval nucleus in their centre.

The size of the corpuscles varies greatly in different classes of animals, but is strikingly constant in the same class. A glance at the following diagram, in which the corpuscles are drawn to