Among the invertebrate animals the globules of the blood are much less regular in their forms. Their surface is uneven and tubercu lated, like that of a raspberry ; their contour is extremely variable; they change their figure with the greatest facility, and their size is considerable. In the blood of the river crab for example (astacus fluviatilis) I have found their mean diameter to be of a line. Several, however, were measured which were no more than of a line across, and others which were as much as In the oyster I have detected still wider differences in the size of the globules of the blood. In the same drop of this creature's blood I found some globules others only and some no more than of a line in diameter.
It is well ascertained that the blood differs during the earlier periods of embryotic existence from what it is in after life. Messrs. Prevost and Dumas have shown that the globules of the blood in the chick in ovo are circular at first, and only become elliptical at the period when the liver is developed.* And M. Prevost found that in the foetus of the goat these corpuscles were at first the double in diameter of those in the adult animal.t The structure of the globules of the blood, as well as their magnitude, has been a subject of great variety of opinion. The differences in the conclusions, however, appear to me to depend principally on the circumstances in the mode of experimenting. Della Torre and Styles believed that the globules of the blood were perforated in the centre and fashioned like rings. When they are examined with lenses of low magnifying power, they look like small black points ; when viewed under an instrument rather more powerful, they assume the appearance of a white circle with a black point in its middle ; this is evidently what has given rise to the opinion we have quoted ; but the appearance in question by no means de pends on the existence of a central hole in the globules ; it is merely the effect of the light ; for by using a magnifying power of 300 or 400, the central point assumes the appearance of a luminous spot, and by varying the position of the globule, as well as the direction of the rays of light, the observer may easily convince himself that the globules are entire. Hewson, to whom we are indebted for so many good observations on the blood, was the first who arrived at accurate conclusions in regard to its globules. He considered them as flattened vesicles, in the interior of which there is a central corpuscle or nucleus. The accuracy of this opinion, which has been maintained in our own day by Messrs. Prevost and Dumas and others, has been called in question by Dr. Hodg kin and Mr. Lister ; nevertheless to me it ap pears to be founded on unquestionable data. In studying the blood of the Reptilia, in which the globules are of very considerable magni tude, Messrs. Prevost and Dumas have even seen the outer envelope of these corpuscles tear, and expose the central nucleus naked. In 1826 I myself observed that by acting with a little weak acetic acid on the globules of the blood, previously placed on the object-plate of a microscope, they are very speedily stripped of their envelope, and their central nucleus is obtained isolated.* Professor Miiller,t who does not appear to have been acquainted with this observation of mine, has lately arrived at the same conclusions, and has varied his ex periments in such wise as to place the results that follow from them in the clearest possible light. I shall only further add that at the
moment of writing this article I have again assured myself of the facts as stated, by sub jecting the blood of the river-crab and that of the frog to renewed examination.
The existence of a solid, white, central nucleus in the globules' of the blood conse quently appears to me to be completely de monstrated; and there is, further, every reason to believe that the peripheries of these cor puscles are membranous vesicles formed of the matter which gives the blood its peculiar colour, or rather that they enclose this colour ing matter between their inner surfaces and the central nuclei. This vesicular part of the glo bule is very elastic : whilst engaged in examin ing the capillary circulation in the lungs of the water-newt, Messrs. Prevost and Dumas frequently saw the globules change their figure under the pressure of the moving column of fluid, and mould themselves in some sort upon the parts that opposed their advance, but they resumed their original form the instant they escaped from the influence of the unequal pressure.t In general the tegumentary vesicle is collapsed upon the central corpuscle, and thus forms a kind of disc of different degrees of thinness near the edges, but plump or filled out towards the middle. By observing the globules of the blood of the frog and water-newt in diffe rent positions, the existence of this central tumi dity may be so positively ascertained as to be be yond the reach of farther doubt ; but in the hu man blood, the globules of which are extremely small and almost entirely occupied by the central nucleus, it is more difficult to be satis fied of its occurrence ; and Dr. Young has even been led to think that these globules are discs concave on both sides, an opinion which has been revived and advocated anew by Dr. Hodgkin and Mr. Lister. The ap pearance or disposition of the globules in question, when it occurs, seems to me to depend on an alteration of these corpuscles. In examining the blood of frogs diluted with thin syrup, the globules occasionally appeared to me to become turgid, but not to be distended equally in every part; the exterior vesicle then remained attached to the centre of the internal nucleus, whilst it became puffed all around. I have seen a globule thus altered in its form, presenting three very distinct en largements in the course of its long diameter, the two lateral of which exceeded the median one in extent. I should therefore be led to imagine that by the effect of an endosmosis these vesicles may occasionally absorb the water of the serum, and that this fluid, accu mulating around the central nucleus, without, however, separating this corpuscle from its envelope, gives to the globule in general the form of a biconcave disc, as described by Dr. Young. This appearance, which is very com mon in the human blood, agrees extremely well with the description of Dr. Hodgkin and Mr. Lister, but we do not imagine that this is the normal condition, and we are persuaded that if these very scrupulous observers would but extend their inquiries to the blood of those animals in which the globules are most easily studied, they would return to and espouse the opinions of Hewson and of Prevost and Dumas in regard to the particular point at issue.