b FIG. 98. to alter their appearance; their outline becomes faint, they are no longer spherical, but very irregular in shape, and constantly change their form by sending out and retracting processes, by means of which they change their position, so that they may be said to perform locomotion. These movements are rendered more active by a slight increase of temperature, and are checked by cold. For continued observation, about 38° C. is the best temperature for mammalian blood. The blood of the frog is generally 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., Vessels of the Frog's Web. (a) Trunk of vein, and (bb) its tributaries passing across the capillary network. The dark spots are pigment cells. even for a considerable time after removal from the body. 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 amoebae (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 proto plasm. 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 nonmammalian 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 scale, will give an idea of their relative sizes, in examples of the different classes of animals, and will make the following points more rapidly obvious than mere description. The size of the animal has no general relation to the size of the corpuscles. The human red discs are of a fair average size when Diagram of the relative sizes of red corpuscles of different animals. The measurements below are in fractions of a millimetre. compared with those of other mammals, and therefore man's blood cannot be distinguished from that of the other mammalia. The mammalian corpuscles are, on the whole, small when compared with those of the other vertebrates. The batrachians are distinguished by the great size of the corpuscles. Those of the Amphiuma tridactylum are visible to the naked eye. The following measurements are given by Welcker for the human discs : Diameter Thickness .0077 of a millimetre (7.7*)= 3200 of an inch. Volume .000000077 of a cubic millimetre. Surface. 12400 of an inch. The last measurement would give a surface of about 2816 square metres for the corpuscles of an adult. A surface of 11 square metres is exposed every second in the lungs for the absorption of oxygen. When circulating in the vessels, or immediately after removal, the red corpuscles are very soft and elastic, being bent and altered in shape by the slightest pressure, and easily stretched to twice their diameter. But the moment pressure or traction is removed, they return to their normal biconcave disc shape if the medium in which they lie continue of the normal density. Changes take place in the blood shortly after it is removed from the body, which seem to be associated with the loss of function (death) of the red discs, as shown by their rapid destruction if reintroduced into the circulation. The changes are checked by cold and facilitated by heat, a temperature above that of the body causing them to take place almost immediately. Associated with the loss of function of the discs is observed a change accompanied by an apparent increase of adhesiveness, which causes them to stick together, commonly adhering by their flat surfaces, so as to form into rolls, like so many coins placed side by side. That this adhesion is not a mere physical process, independent of the chemical properties of the corpuscles themselves, seems proved by the following facts: (1) It does not occur immediately when the blood is drawn, and disappears after a few hours without the addition of reagents; (2) while the blood is in the living vessels under normal conditions there is no adhesion, but this soon appears when * The Greek letter μ is used by histologists to denote ross of a millimetre, which is taken as a convenient unit of measurement. |