BLOOD. 



405 



and the best connected series of inquiries into 

 the composition and qualities of this fluid.* 



The form of the globules of the blood varies 

 in different animals, but appears to be at all 

 times essentially the same in individuals of 

 the same species ; this at least is the case if we 

 except the first periods of their embryotic 

 existence ; for in the embryo the globules have 

 been found to be different before the formation 

 of the liver, from what they are after the deve- 

 lopment of this organ. 



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 

 modem 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 of which 

 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. f 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, J 

 was, however, completely refuted by Hewson, 

 Prevost and Dumas, Ilodgkin and Lister, 

 Miiller, &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 

 link that the whole of the globules of the 

 blood of any animal are of the same dimen- 

 When blood in which these globules 

 are very minute is examined, and a low mag- 

 lifying 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, 

 have satisfied myself that they differed in size 



Vide Hewson on the Blood, and Prevost and 

 Dumas, Examen du Sang et de son action dans les 

 diverses phenomenes de la Vie, iu Biblioth. 

 IJnivers. de Geneve, t, xvii. 



t Philos. Trans. No. 165. 1684. p. 788. 



J Ibid. 1818. 



in the same individual. Among the lower 

 animals, the river-crab (astacusjluviatilis) 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 the 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 1 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. 



These estimates accord very nearly with the 

 last admeasurements published by M. Dumas, 

 and taken by a different method. He gives 

 31 ten thousandths of a line as the mean dia- 

 meter of a globule of the human blood accord- 

 ing to his latest observations.* 



The conclusions come to by Dr. Hodgkin 

 and Mr. Lister are also very nearly the same, 

 these observers estimating the diameter of the 



* Annales des Sciences Nalurelles, tom. xii. 

 p. 59. 



