Howson thought that the fibrin became specifically fighter, and the red particles, of course, comparatively heavier, whence they were disposed to fall to the bottom of the clot. Hunter seemed disposed to account for the appearance from the firmer coagulation of the fibrin, as it were, squeezing out the red particles; but this would nor explain why the upper part of the clot alone is left with out them. Hey's opinion seems to be better founded ; that by the increased action of the vessels, the different constituents are more intimately united together, the fibrin therefore requires a longer space of time for its coagula tion, and thus the red particles have an opportunity of par tially subsiding. It is, however, not improbable that Hun ter's opinion is also in part correct ; for we find that the clot of inflamed blood obviously possesses a firmer texture than it exhibits in its ordinary state, thus affording an ad ditional analogy between the coagulation of the blood and the contractility of the muscles ; for it appears, that in this state of fever the force of muscular contraction in the organs of the circulation is augmented in the same pro portion with the coagulating powers of the fibrin.
The red particles, which together with the fibrin com pose the crassamentum, from the peculiarity of their ap pearance and organization, have been the subject of almost innumerable observations and experiments. Soon after the introduction of the microscope into physiological re searches, these substances were minutely examined by Malpighi, and afterwards by the indefatigable Leeuwen hoek. They were at first described simply as spherical bodies, floating in the serum, and giving the blood its red colour ; but as observations were multiplied, errors and absurdities were advanced in almost an equal proportion. Lceuwcnhoek himself invented a fanciful hypothesis, which had a long and powerful influence over the most en lightened physiologists, that the red particles of the blood were composed of a series of bodies descending in regular gradations; each particle was supposed to be made up of six particles of serum, a particle of scrum of six particles of lymph, &c. This hypothesis, for which there does not appear to be the slightest foundation, was so suited to the mechanical genius of the age, that it was generally adopt ed, and even formed a leading feature in all the doctrines of Boerhaave. Its futility was exposed, in the first instance, by Lancisi and Senac, but it still maintained its ground until the time of Haller.
When objects can only be detected by lenses of highly magnifying powers, it is so extremely difficult to avoid being misled by ocular deceptions, that all descriptions of this kind are to be received with the greatest caution. The necessity for this caution is sufficiently proved by the dis cordant accounts of the red particles that have been given by the different observers, who, as far as we can judge, could have no motive for intentionally imposing upon their readers. Hewson published a very elaborate description
of these bodies professing to be the. result of accurate observation ; he informs us that they consist of a hollow vesicle, containing a solid body in its centre. The Abbe Torre described the red particles as being in the form of rings, while Monro thought that they were flattened bo dies, in the shape of coins, with a dark spot in the centre. Cavallo, after reviewing the opinions of those who had preceded him, concluded that they were all deceptions, owing to the peculiar modification of the rays of light, and imagines that the real figure of the particles is a sphere, while Dr. Young, from the result of his observations, has been induced to adopt an opinion nearly similar to that of Hewson. The particles of the blood of the skate, from their size and distinctness, arc the most proper for obser vations of this kind; and he announces them as being composed of an external envelope, containing a central nu cleus, the two bodies having but little adherence to each other, and seeming to be of a very different nature and consistence. Dr. Young has also observed in the globules of the human blood an appearance, which partly coincides with that stated by Monro, as they arc said to be flattened and to have a depression in the centre.
Very various statements have been made respecting the size of the red particles ; Leeuwenhoek says it differs much in different animals; but later writers have thought that they are more nearly of the same size. Haller estimat ed them at about the five-hundredth of an inch in diame ter, and this estimate is sanctioned by the observations that have been more recently made by Dr. Young and Captain hater; while, on the contrary, the still later re searches of Mr. Bauer make than considerably larger, as much as the two-hundredth of an inch in diameter.
The nature and composition of the particles have been the subject of nearly as much controversy as their figure. The older writers supposed them to be of an oleaginous nature, but this idea is incompatible with the more cor rect observations that have been since made upon them, according to which they bear no resemblance to an oily body, either in their physical or their chemical properties. A part at least of the uncertainty which attaches to their nature depends upon the difficulty which there is in pro curing these bodies in a separate state, detached from the other constituents of the blood ; this objection applies to the analysis of Berzelius, although it is probably the most correct that we possess. The result of his experiments is, that the red globules do not materially differ from the other parts of the blood, except in their colour, and in the circumstance of a minute quantity of oxide of iron being found among their ashes after combustion.