The experiments of Berzelius have shown that the serum of the blood of the ox does not differ essentially from that of the blood of man.t But we are still without comparative analyses of the nutrient fluids of the different classes of animals. This desideratum has been partially supplied in regard to the vertebrata by Messrs. Prevost and Dumas, they having carefully determined the proportions of water, and of albumen contained in the serum, and those of the fibrine, and other solid parts which swim suspended in this fluid. From these expe riments we learn that the composition of the serum varies in the same animal at different times, and that it differs still more widely in different animals, without its being possible to connect such changes with the physiological state of the individual. The case is otherwise, however, as concerns the globules ; in the majority of cases there exists a remarkable relation between the quantity of these cor puscles and the degree of heat developed by the vital actions. Of this we may be easily convinced by inspecting the following table, in which Messrs. Prevost and Dumas have pre sented us with the comparative weights of the solid particles (globules and fibrine) contained in 1000 parts of blood, with the habitual temperature of different animals, taken in the rectum, the number of pulsations of the heart per minute, and the number of inspirations made in interval of time.
From these experiments it follows that of all animals birds are those whose blood is richest in globules and in fibrine, as they are those also whose temperature is highest and whose respiration is most active. The blood of the mammalia contains rather less, and there is a difference to be noted in this respect between the carnivorous or omnivorous tribes, and the herbivorous, the proportion of solid paiticles being larger in the two former than it is in the latter. We see, indeed, that in man, the dog, and the cat, they enter in the proportion of twelve or thirteen per 1000, whilst in the horse, sheep, calf, and rabbit, they form no more than from the seventh to the ninth per 1000 of the general weight of the blood. But the number of species hitherto examined is not so considerable as to enable us to say that the circumstance, now announced, is to be regarded in the light of a physiological law. Among the cold-blooded vertebrate animals the blood becomes much poorer in solid particles; the tortoise, indeed, seems, from the results in the table, to form an exception to this fact, but the circumstances under which the estimates were made in regard to it, and which it would he too long to enter upon here, explain the anomaly.* The proportion of serum and of solid parti cles also presents considerable varieties in the blood of different individuals of the same species. From the investigations of M. Lecanu we observe that the proportion of water in the human blood varies from 853 to 778 in 1000, and that of the solid particles from 148 to 68.
The differences of sex have also a certain Lastly, the composition of the blood may also vary in the same individual according to a variety of circumstances. Prolonged absti
nence from diluents, for example, tends to di minish the proportion of the watery particles of the blood, and, consequently, to render it richer in nutrient elements. Bloodletting pro duces the contrary effect; not only is the mass of circulating fluid by this means diminished, but it is also rendered poorer. Messrs. Prevost and Dumas having bled a cat largely, found its blood to consist of 791 of water, 87 of albumen, and 118 of globules. Two minutes afterwards they repeated the bleeding, and now only found 116 of globules, and 74 of albu men to 809 of water ; after an interval of five minutes more the bleeding was repeated for the third time, and they found the blood to consist of 829 of water, 93 of solid particles, and 77 of albumen. M. Lecanu obtained similar results from the analysis of human blood taken from patients who had been bled several times in quick succession, or who were labouring under hcemorrhagic affections ;* and the circumstance is readily explained, by sup posing that the diminution of the mass of blood tends to accelerate absorption, the first effect of which must needs be to introduce a much larger proportion of water than of solid particles into the torrent of the circulation.
In its ordinary state the blood is always fluid, and consists, as we have seen, of a watery part, holding solid globules in suspen sion; but under certain circumstances itsphysical properties change completely: this happens whenever it is withdrawn from the vessels in which it is contained in the bodies of living animals, or in the event of an animal ceasing to exist. The blood left to itself changes within a few minutes into a mass of a gelatinous con sistence, which gradually separates into two parts, one fluid, transparent, and of a yel lowish colour, formed by the serum; another solid, quite opaque, and of a red colour, to which the name of cruor, erussumentum, or clot is given.
The mode in which this phenomenon hap pens, and the cause that occasions it, have engaged the attention of a great many physio logists. The experiments of Hunter and of many others show that the coagulation of the blood depends mainly on the cessation of the motion to which it is constantly subjected in the course of the circulation ; for this condition alone suffices to make it coagulate even in the interior of the vascular system, and we are of opinion that the great physiologist just quoted erred in attributing vital properties to the blood. Rest, then, cessation from motion, is that which contributes most generally and most essentially to cause coagulation of the blood; other circumstances, however, such as its cooling, its being brought into contact with the air, &c. may also contribute to accelerate this phenomenon, which appears, from the experiments of Dr. John Davy, to be unac companied with any evolution of caloric.