As the aggregate diameter of the vessels in the body decreases, the blood in them flows with greater rapidity and force; and when near the heart, it is placed in circumstances unfavourable to coagulation, in consequence of the rapid motion there communicated to it; and we accordingly find that, although adherent coagula may be formed in the veins leading from the seat of injury, and may be traced thence through other larger vessels, they will usually terminate abruptly, when these open into the vena cava. But when the blood arrives in the cavities of the heart, the tendency to coagulate may again manifest itself. Diseased coagula, presenting a "mottled appearance, partly brown, and partly of a dirty yellow colour", or "dark coloured, and partly composed of a yellowish grey opaque substance", or "with portions inelastic, and of an opaque yellow colour", will be found entangled among the projecting fibres of the auricles and ventricles. In the arteries, the blood is in the most unfavourable circumstances for coagulation during life, in consequence of the comparative smallness of their diameters, and the succession of impulses communicated to their contents; but even here, diseased and adherent coagula may be found in cases of secondary inflammations.
M. Cruveilhier[37] relates an instance in which, after child-birth, the pulmonary artery was found filled with coagula following its divisions. The principal clot had lost its colour, and was adherent to the sides of the vessel, and contained in its centre puriform fluid.
In another case, following the operation for necrosis, the following post-mortem appearances were observed. The left lung presented, in several places, patches of red hepatization, perfectly defined, and resembling so many spots of lobular inflammation. When cut into, these patches presented several puncta of puriform fluid. The veins of the lung contained fibrinous-looking coagula blocking up their cavities; and in the centre of these coagula was a whitish purulent looking fluid.
The diseased condition of the blood may thus be traced visibly from the original wound, through the larger vessels, to the heart, and again from the heart to the capillary system. The disposition to coagulate, once impressed upon the blood, is not destroyed by that fluid being conveyed to a different part of the body: the action may be retarded by motion in, as well as out of the body, but will nevertheless occur when it is placed under more favourable circumstances.
These circumstances, in the living vessels, are when the blood becomes separated into small quantities, and when it moves slowly along the capillaries. The blood will then coagulate in circumscribed patches,[38] as illustrated in the first characteristic marks of secondary disease which have already been mentioned as occurring in the lungs, the liver, the spleen, and the skin. The accompanying plate is taken from the lung of a donkey in which purulent fluid had been made to circulate with the blood. The stagnation of the blood, when the pus was first introduced, was mechanically prevented, and the livid spots produced by its subsequent coagulation in the capillaries of the lungs has been very faithfully represented. (See Experiment No. [vi].)
"Besides the disposition for coagulation," observes Mr. Hunter, "the blood has, under certain circumstances, a disposition for the separation of the red globules, and probably of all its parts; for I have reason to believe, that a disposition for a separation of the red part and coagulation, are not the same thing, but arise from two different principles. This is always observable in bleeding; for if we tie up an arm and do not bleed immediately, the first blood that flows from the orifice, or that which has stagnated for some time in the veins, will soonest separate into its three constituent parts: this circumstance exposes more of the coagulating lymph at the top, which is supposed by the ignorant to indicate more inflammation, while the next quantity taken suspends its red parts in the lymph, and gives the idea that the first small quantity had been of such service at the time of its flowing, as to have altered for the better the whole mass of blood. Best, therefore, maybe regarded as one of the immediate causes of the separation." Hunter, p. 29.
This disposition of the blood to separate into its constituent parts is evinced in a very marked degree in one class of secondary affections. Extensive effusions of serum, lymph, and pus, mixed in different proportions, will take place in the serous cavities of the body, and become infiltrated in the cellular membrane, accompanied with very slight indications of inflammatory action. The colouring matter of the blood will also sometimes become effused with its other parts; but when this is the case, the blood will be found to have lost its coagulating power: in this respect presenting a direct contrast to the effusion from a healthy wounded vessel. The lymph deposited will be found lying in unorganized flakes, wanting its usual adhesive properties, and very slightly attached to parts, presenting little or no increased vascularity. The rapid manner in which these depositions take place, shows that they are separated from the blood without undergoing any very elaborate process. In this condition of the system, any organ upon which the disease falls, may rapidly become disorganized, or may readily mortify; and, after death, a tendency to rapid decomposition will be manifested. The veins on the surface of the body may frequently be traced as dark blue lines, as though the skin covering them were stained by the colouring matter of the blood. The lungs and other organs may, under these circumstances, be found in every grade of disorganization, till they present all the characteristics of gangrene: even the peculiar fetor which accompanies mortification of the lung, will, in some instances, be present. A tendency to the formation of petechial spots may also be observed in different parts; and even the organs which do not appear to have been the peculiar seat of the disease, will be found to have lost their consistency, and to break down upon comparatively slight pressure.
Mr. Hunter found, that in proportion as the blood retained the power of coagulation, it had the power of resisting putrefaction; and conversely, we observe that, in this class of cases, the deficiency of the former is accompanied in a marked manner by the absence of the latter.
The two conditions of the blood which have now been mentioned, appear to bear a direct relation to the two classes of post-mortem appearances, observed in cases of secondary inflammations: the first being generally connected with congestion of different organs during the first stages of the disease, the second with extensive effusions, accompanied with comparatively little vascularity.