CHANGES THAT OCCUR IN MATION.
Changes in the Blood-vessels in inflam mation.
To understand the nature of these changes it is necessary to recall the characters of the blood and of its movement in the blood-vessels, as seen under the microscope.
We remember that the blood contains red and white cells or corpuscles (p. 292), the white being few in comparison with the red ; and that, when the circulation of blood is seen, say in the web of a frog's foot, the blood is observed to flow in an even stream in the capillary blood vessels, easily and uniformly, the current of blood corpuscles—red and white mingled— passing along the centre of the vessel, and not adhering to its walls.
Now if the part being watched under the microscope be irritated or injured in any way, at once a remarkable change takes place in the vessels. They widen, and an increased quantity of blood flows to them. Blood-vessels but faintly perceived before the injury are now very plain because of their increased size, and others for merly invisible from their narrowness now start into view. There is manifestly a much larger quantity of blood in the part, and this increase in quantity is accompanied by its signs of increased redness and warmth. At first also the current of blood is not only fuller but faster, and the part may throb with the fulness and speed of the stream. But soon the blood begins to flow less quickly through the vessels; the current is delayed. To this stage in the process of inflammation the term con gestion is applied. This is not yet Mamma tiou. If the irritation has been slight, or has been quickly removed, the excessive quantity of .blood may pass away, the vessels gradually recover their former size, and no signs of any unusual occurrence remain.
But if the irritation be great, or continue, the process goes on. As a result of the fulness of the blood-vessels, and the pressure exerted by the blood within them on their walls, leakage takes place, and fluid passes out of the vessels into the surrounding tissues. On this account,
as well as because of the large quantity of blood in the part, some degree of swelling is apparent. It is this exudation, as it is called, that goes to form the fluid found in quantity in such serous cavities as that of the pericardium, surrounding the heart, or that of the pleurae, surrounding the lungs, when these membranes are inflamed, as in dropsy of the heart (p. 318), and pleurisy (p. 359).
Accompanying such changes is another of remarkable importance. The observer, watch ing carefully through his microscope the effects of irritating the frog's web, perceives that, with the slowing of the current, the white blood corpuscles quit the centre of the stream and begin to loiter along the walls of the ves sels. So that instead of a central stream of corpuscles in the vessels, with clear margins, there is a central stream of red corpuscles, but a row of white corpuscles on each side, which soon cease to roll along, and adhere to the walls. They do more than this. The white cells send out processes which pierce the fine walls of the vessel, and gradually these processes grow till the cella themselves are found bodily trans ferred to the outside of the vessel. The curious thing is that no opening can be discerned through which the cells could pass, but, all along the capillaries of the inflamed district, white cells are seen either lying outside of the vessels or in various stages of their passage through the walls. A school-boy will blow a soap - bubble, and will pass peas and coins through its walls without rupturing them or destroying the bubble; and we can only sup pose that, in a similar way, the white cells pass through thin blood-vessel walls without any breach of substance. in severe attacks, red cells also escape from the over-full vessels into the surrounding tissue. Fig. 146 illustrates the appearances of a piece of tissue in which these occurrences are taking place.