144 



ON THE CIRCULATION OF THE BLOOD, 



But the new-formed fluid, even at the time it has reached the lieart,ha?3 

 by no rneans undergone a sufficient elaboration to become genuine blood, 

 or to support the living action of the different organs. It has yet to be 

 operated upon by the air, and must for this purpose be sent to the lungs, 

 and again returned to the heart, before it is fitted to be thrown into the 

 general circulation. 



This is the rule that takes place in ail the more perfect anmjals, as 

 mammals, birds, and most of the amphibiais and hence these classes are 

 flaid to have a double circulation. And as the heart itself consists of four 

 cavities, a pair belonging to each of the two cncuialions, and each paifr is 

 divided from the other by a strong rnembrane, they are also said to have 

 not only a double circulation, but a double heart — a pulmonary and a cor- 

 poreal heart. 



The blood is first received into the heart on the pulmonary side, and is 

 conveyed to the lungs by an artery which is hence called the pulmonary 

 artery, that soon divides into two branches, one for each of the lungs ; in 

 which organs they still further divide into innumerable ramifications, and 

 form a beautiful net-work of vessels upon the air-vesicles of which the sub- 

 stance of the lungs consists ; and by this mean every particle of blood is 

 exposed in its turn to the full influence of the vital gases of the atmos- 

 phere, and becomes thoroughly issnnilated to the nature of the animal 

 system it is to support. The invihibly muiute arteries now terminate in 

 equally minute veins, which progressively unite till they centre in four com- 

 mon trunks, which carry back the blood, now thoroughly ventilated and 

 of a florid hue, to the left side or corporeal department of the heart. 



From this quarter the corporeal circulation commences : the stimulus of 

 the blood itself excites the heart to that alternate contraction which con- 

 stitutes pulsation, and which is continued through the whole course of the 

 arteries ; and by this very contraction the blood is impelled to the re- 

 motest part of the body, the arterial vessels continuing to divide and to 

 subdivide, and to branch out in every possible direction, till the eye can 

 no longer follow thexn, even when aided by the best glasses. 



The arterial blood having thus visited every portion of every organ, and 

 supplied it with the food of life, is now returned, faint, exhausted, and of 

 a purple hue, by the veins, as in the pulmonary circulation ; it receives, a 

 short space before it reaches the heart, its regular recruit of new matter 

 from the digestive organs, and then empties itself into the right side or 

 pulmonary department of the heart, whence it is again sent to the lungs, 

 as before,' for a new supply of vital power. 



The circulation of the blood, therefore, depends upon two distinct sets of, 

 vessels, arteries, and veins, the former of which carry it forward to every 

 part of the system, and the latter of which return it to its central source. 

 Both sets of vessels are generally con iidered as consisting of three distinct, 

 layers or tunics ; an external, which in the arteries is peculiarly elastic ; a 

 middle, which is muscular in both, but whose existence is doubted by some 

 physiologists ; and an internal, vvhicii may be regarded as the common 

 covering or cuticle. The projectile power exercised over the arteries is 

 unquestionably the contraction to which the muscular tunic of the heart is 



* Cuvier seems to ascribe a double heart to the class of amphibia, without any limitation- 

 See Lawrence's additional note E. chap. xii. of his translation of Blumenbach's System of 

 Comparative Anatomy. Blumenbach himself has remarked, that many of Hjte frogs, lizards, 

 and serpents have a simple heart, consisting of a sinde auricle and ventricle, like that of 

 fishes. Sect. 162. i= ^ 



