PROPERTIES OF THE BLOOD. 433 



puscles are generally larger in the embryo than in the adult, especially soon 

 after the period of their first formation ; it was remarked by M. Prevost that 

 in the foetal goat they were at first twice the size of those of the mother. Mr. 

 Gulliver has observed however, that at a later period of utero-gestation they 

 are sometimes smaller than the average dimension of the adult : but perhaps 

 all such observations are to be received with hesitation, owing to the fact 

 mentioned by him, that the variety in the magnitude of the foetal corpuscles is 

 much greater than in the full-grown animal. 



576. In regard to the uses of the red Blood-corpuscles in the animal econ- 

 omy, it appears to the Author that a definite conclusion may be now arrived 

 at. Their existence in the circulating fluid is confined to the Vertebrated 

 classes ; the corpuscles which are seen in the blood of the Invertebrata being 

 analogous rather to the colourless corpuscles, presently to be described as 

 present in the blood of the higher animals. Hence the inference appears 

 irresistible, that they are not essentially necessary to the production of the 

 organizable elements of the blood, or of the organized tissues. The red cor- 

 puscles are most abundant in those classes among Vertebrata, which maintain 

 the highest temperature ; thus, they are somewhat more numerous, in propor- 

 tion to the whole bulk of the blood, in Birds than in Mammalia ; and far more 

 in the latter than in Reptiles and Fishes. As it is evident that they undergo 

 very important changes in the pulmonary and systemic capillaries, their 

 colour being changed from purple to red in the former, and from red to purple 

 in the latter; it seems highly probable that they have for their principal 

 office the introduction of oxygen into the blood that circulates through the 

 systemic capillaries, and the removal of the carbonic acid set free there ; 

 to serve as the medium, in fact, for bringing the tissues into relation with the 

 air, the influence of which is necessary for the maintenance of their vital 

 activity. In the Invertebrata generally, whose respiration is very feeble, this 

 end will be sufficiently answered by the fluid plasma of the blood ; the altera- 

 tions in which, under the influence of the air, have been already noticed ( 540 

 and 553). And in Insects, the only class whose respiration is at all active, 

 we find the air directly conveyed into the tissues; the circulating fluid not 

 being employed as its carrier. By Liebig it is supposed, that the iron in the 

 red corpuscles is the real agent in the respiratory process ; for if its original 

 state be the protoxide, it may become the peroxide by uniting with an 

 additional atom of oxygen, or the protocarbonate by the addition of an atom of 

 carbonic acid. The former change is supposed by him to take place in the 

 lungs, to which the blood comes charged with carbonic acid ; the carbonic 

 acid is given up by the iron, and replaced by an equivalent of oxygen taken 

 in from the air; whilst in the systemic capillaries, the converse change 

 takes place, the oxygen being imparted to the tissues, and being replaced 

 by carbonic acid which is given up by them to be conveyed out of the system. 

 It is stated by Liebig that there is far more than sufficient iron in the whole 

 mass of the blood, to convey in this manner all the oxygen and carbonic acid, 

 which are interchanged between the pulmonary and systemic capillaries. 

 The speculation is certainly an ingenious one ; but it can scarcely be yet 

 received as a physiological fact. In addition to their uses in the respiratory 

 process, it would not seem unlikely that the red corpuscles may be of impor- 

 tant assistance in promoting the movement of the fluid in which they are 

 suspended ; for, if it be true that this partly depends upon the chemical con- 

 dition of the blood in respect to the tissues which it supplies, any attractions 

 or repulsions arising out of this may be more powerfully exercised upon a 

 solid corpuscle than upon the constantly-shifting particles of the fluid. Hence, 

 perhaps, the local congestions of anaemic patients. 



577. Besides the red particles of the Blood, there are others which possess 

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