RESPIRATION, AND ANIMAL1ZAT1ON. 139 



towards establishing the healing art upon a rational basis, and subjecting the 

 different diseases of mankind to a successful mode of practice, than any 

 other discovery that has emblazoned the annals of medicine. 



In our last lecture we traced the action of the digestive organs : we beheld 

 the food first comminuted by means of jaws, teeth, or peculiar muscles or 

 membranes ; next converted into a pulpy mass, and afterward into a milky 

 liquid ; and in this state drunk up by the mouths of innumerable minute 

 vessels, that progressively unite into one common trunk, and convey it to 

 the heart as the chief organ of the system, for the use and benefit of the 

 whole. 



But the new-formed fluid, even at the time it has reached the heart, has by 

 no means 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 re- 

 turned to the heart, before it is fitted to be thrown into the general circulation. 



This is the rule that takes place in all the more perfect animals, as mammals, 

 birds, and most of the amphibials ;* and hence these classes are said to have a 

 double circulation. And as the heart itself consists of four cavities, a pair 

 belonging to each of the two circulations, and each pair is divided from the 

 other by a strong membrane, they are also said to have not only a double 

 circulation, but a double heart a pulmonary and a corporeal heart. 



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

 veyed 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 farther divide into innumerable ramifications, and form a 

 beautiful network of vessels upon the air vesicles of which the substance 

 of the lungs consists ; and by this means every particle of blood is exposed 

 in its turn to the full influence of the vital gases of the atmosphere, and be- 

 comes thoroughly assimilated to the nature of the animal system it is to 

 support. The invisibly minute arteries now terminate in equally minute 

 veins, which progressively unite? till they centre in four common 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 thelieart to that alternate contraction which constitutes 

 pulsation, and which is continued through the whole course of the arteries ; 

 and by this very contraction the blood is impelled to the remotest 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 them, 

 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 de- 

 partment 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 considered 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, which may be regarded as the common covering 

 or cuticle. The projectile power exercised over the arteries is unquestion- 

 ably the contraction to which the muscular tunic of the heart is excited by 



* Cuvier seems to ascribe a double heart to the cla.-vs of amphibia, without any limitation. See Law- 

 rence's additional note E. chap. xii. of his translation of Blumenbach's System of Comparative Anatomy. 

 Blumenbach himself has remarked, that many of the frogs, lizards, and serpents have a simple heart, 

 consisting of a single auricle and ventricle, like that of fishes Sect. 162. 



