The globules of the blood of all the mam malia that have been examined are of a circu lar shape, whilst in birds, reptiles, and fishes, they are elliptical; in the invertebrate animals, however, they are again circular.
The whole of the microscopical observers of modern times are agreed in the above points; but they differ in opinion with regard to the nature of these bodies. This discrepancy, however, does not appear to us to be owing so much to any optical illusion to which the microscope exposes those who use it, as to the choice of objects made by the different observers. Too many of them have been satisfied with the study of the human blood, the globules ofwhich are extremely small and always seen with great difficulty, whilst, had they made use of the blood of certain animals, as of the frog, or, bet ter still, of the water-newt ( Salamandra cris tata), they would have escaped much of the uncertainty that surrounds their conclusions.
The globules of all animals having red blood are more or less flattened, and in the greater number of cases they resemble a small circular or elliptical disc. Leuwenhoeck was aware of this fact in reference to birds, reptiles, and fishes, but he believed that in the human sub ject and the other mammalia these bodies were spherical.t This error, which was sanctioned by Fontana and various others of the older observers, and has even very recently been adopted by Sir E. Home and M. Bauer,t was, however, completely refuted by Hewson, Prevost and Dumas, Hodgkin and Lister, Muller, &c.; my own observations also con firm the conclusions of these physiologists, and even go to prove that the globules of the blood in the invertebrata have the general form of flattened vesicles.
The greater number of observers appear to think that the whole of the globules of the blood of any animal are of the same dimen sions. When blood in which these globules are very minute is examined, and a low mag nifying power is employed, it is quite true that no perceptible difference in point of size can be detected ; but by estimating the mag nitudes of a great number of these globules com paratively and under a powerful microscope, I have satisfied myself that they differed in size in the same individual. Among the lower animals, the river-crab (astacus fluviatili,$) for instance, it is by no means difficult in the same drop of blood to perceive globules of very dif ferent dimensions ; and although this inequality is much less remarkable among the higher ani mals, I may affirm that it exists. Thus, in the
same drop of a frog's blood, I have seen globules that differed from one another in the proportion of 39 to 45, without my being able to ascribe these variations of diameter to any circumstance connected with my mode of observing, or to any optical illusion : the globules were spread in a single layer upon the object plate, and so close together as to be exactly within ;he focus of the instrument. Their apparent diameters, I may state, were estimated by tracing, with the assistance of the camera lucida, the out lines of their images, upon a board eight inches distant from the eye-piece. I found corresponding differences of dimension among the globules of the human blood ; in the same drop I have measured several which were to each other in the ratio of 112 to 140 : in ge neral, however, the differences are scarcely appreciable.
The globules of the blood appear to be identical in every part of the circulating sys tem, and hitherto no difference in their size or shape has been detected in individuals of the same species, though of different ages and sexes.
It was long found a matter of considerable difficulty to determine the precise diameter of the globules of the blood ; we consequently find marked discrepancies in the conclusions come to by different microscopists. At the present day, however, and since our means of observation have been improved, the estimates have become gradually less and less discordant, and therefore may be held worthy of the greater confidence.
From a very great number of measurements taken by means of the process of M. Amici (with the camera lucida), and under a mag nifying power of 900, I have obtained as the mean term of the diameter of the globules of the human blood of a line (English,) or in decimal fractions 0,00030 of a line. But as I have already said, I have found considerable variety in the sizes of the globules; some, and these were the largest, were 35 ten thou sandth parts of a line, and others, the smallest, no more than 28 ten thousandths of a line in diameter.