The resemblance of the cells met with in acute inflammations to the leucocytes in the blood long ago suggested the idea that they are white blood corpuscles. In the year 1846 Waller observed the pavementing of the internal coat of the veins with white blood-corpuscles in the inflamed tongue of the frog, and as he saw similar cells outside the vessel he inferred that the white corpuscles had got through the wall. It was, however, difficult to believe that the solid globular white corpuscles could pass through the intact wall of a blood-vessel, and Waller's views, although supported by William Addison, were lost sight of. The discovery by Recklinghausen that pus corpuscles and white blood-corpuscles possess contractile power by virtue of which they are able to move from place to place, and to alter their shapes in the most diverse fashion, paved the way for the actual observation of their passage through the walls of the vessels made by Cohnheim.
This was observed by Cohnheim in the mesentery of the frog. When this exceedingly delicate and transparent structure is drawn out of the bod}' through a wound in the lateral aspect of the abdomen, the mere exposure to the air is sufficient to set up an acute inflammation, the phenomena of which can be readily observed under the microscope. Let us suppose that the pavementing of the veins has occurred, and that there is an occasional white corpuscle adherent in the capillaries, and the following surprising phenomena show themselves, as described by Cohnheim himself. The various steps are represented in Fig. 60, in which the red corpuscles are left out in order to bring the leucocytes into prominence. "One sees, as a rule, first in a vein which presents the pavementing with white corpuscles, but sometimes in a capillary, a pointed projection in the external contour of the vessel; it pushes itself farther and farther outwards, it increases in thickness, and the pointed projection develops into a colourless rounded knob; this increases in length and thickness, sends out fresh points, and draws itself gradually outwards from the vessel-wall, with which it comes to be connected only by a long thin stem. Finally, this also lets go the vessel, and there lies outside a colourless, dull, glancing, contractile corpuscle, with several short processes and a long one, of the size of a white blood cell, with one or several nuclei; in a word, a white blood-corpuscle".
Fig. 60. - Diagram showing emigration of leucocytes in a capillary vessel. (After Arnold).
This is a frequent accompaniment of acute inflammations. The white corpuscles are active contractile cells, and they generally pass through the vessels to a larger extent than the red ones, but there are some inflammations in which the red corpuscles also pass through in large numbers. They are not active, but are passively pushed through the walls, just as they are in the diapedesis of passive hyperemia.
The red corpuscles and the leucocytes are so different in their characters that the same explanation can hardly apply to both. The red corpuscles are devoid of nuclei and possess no spontaneous movement. Hence their exudation, in inflammation as in passive hyperemia, is a passive process. It is produced by pressure from within, and the altered condition of the vessel wall, as well as of the circulation, is an important element in its causation. That the state of the vessels is an important factor is evidenced by the fact that exudation of the red corpuscles is specially met with in the more severe forms of inflammation. As the red corpuscles are the coloured elements of the blood, inflammations in which they are specially exuded have a more or less haemorrhagic character, and it is generally recognized that such conditions as haemorrhagic pneumonia and haemorrhagic small-pox are specially virulent forms of disease.
The leucocytes being active bodies have not such a merely passive part to play. They are amoeboid cells, and pass out of the vessels by virtue of their inherent contractility. No doubt the altered state of the vessel wall renders it easier for the leucocytes to pass through. The leucocytes probably do not pass through the midst of the endothelial cells of the vessel wall, but between them. According to Arnold both the red and white pass out largely through pre existing apertures or stomata. (See Fig. 25, p. 89, for the red, and Fig. 60 for the white.) As the corpuscles pass through apertures much narrower than themselves they undergo alterations in shape, so that the part actually in the wall is very attenuated.
As it is a characteristic feature of most acute inflammations that the leucocytes pass out of the vessels, and as they do so often in extraordinary numbers, some explanation of this fact must be sought. Light has been thrown on the matter by the researches of Metchnikoff and others. From the observation of unicellular organisms having the form of amoebae, it appears that certain substances in solution are attractive to these bodies and others are repulsive. As this depends on the chemical nature of the substances, the terms positive and negative chemiotaxis have been introduced by Pfeffer. Thus a decoction of dead leaves attracts the plasmodia, whilst solutions of salt, sugar, and other substances repel them. As the leucocytes of the blood seem to be of a similar nature, it is not surprising that they also are liable to repulsion and attraction by different substances. The extraordinary migration of leucocytes in many inflammations can scarcely be otherwise explained.
For example, Fig. 61 represents a lesion of the skin (Sudamen), in which leucocytes have accumulated in a dilated sweat-duct. In order to reach this position they have not only penetrated the vessel wall, but insinuated themselves amongst the epidermic cells till they reached the interior of the duct. There seems no way of explaining this but by the view that the sweat contained some substance attractive to the leucocytes. It has diffused from the duct so as, in a dilute form, to reach the vessels and attract the leucocytes which have passed in the direction of its more concentrated presence.
The corpuscles which leave the vessels in inflammations are the polymorphonuclear or neutrophil leucocytes. In stained sections the presence of leucocytes of this character can often be made out either in the tissue or along with the fibrine, as in Fig. 62. It has been suggested that this lobed character of the nucleus may be in order to allow of emigration, as the solid bulky nucleus of the other forms would have great difficulty in getting through the minute stomata (Metchnikoff).
This attractive power of some substances is supposed to have a special meaning, and the emigration of the leucocytes comes to occupy a central position in the most recent theory of inflammation, that of Metchnikofl'. The amoeboid cells of the various orders of animals have undoubtedly the power of englobing, and sometimes of digesting, solid granular matter. This power has relation either to the nutrition of the cell or to the protection of the organism. The cells may englobe foreign deleterious granules, and more particularly parasitic microbes, and even by destroying them rid the body of them. This process of protective phagocytosis is stated to be the explanation of the exudation of the leucocytes. It is generally a case of infection with microbes, and it is suggested that the chemical products evolved by the microbes attract the leucocytes, which are induced to leave the vessels and encounter the microbes.
Fig. 6l. - Sudamen formed by dilated sweat-duct. It is largely occupied by leucocytes attracted thither. X80.
It is consistent with this view of the attractive power of certain substances that some inflammations are associated with little or no emigration of leucocytes. Thus we have serous inflammations in which there are few leucocytes. In inflammations produced by the bacillus of malignant oedema also there is abundant serous exudation, and frequently abundant red corpuscles, but the leucocytes are few. The poison produced by this microbe exercises apparently a negative chemiotaxis on the leucocytes.