CIRCULATION OF THE BLOOD. 



497 



arteries, thus closely approaching the double circulation of birds and 

 mammals (Fig. 190). A portal circulation is also present in the cold- 

 blooded pulmonated vertebrata, and, as in the fishes, is connected with 

 the renal veins. 



In birds the heart, as in man, consists of four cavities, two auricles 

 and two ventricles, and the general distribution of the circulation 

 is the same, i.e., the right auricle collects the blood from the sys- 

 temic veins and transmits it to the right ventricle, which, by means of the 

 pulmonary artery, forces the blood through the lungs. From the lungs the 

 oxygenated blood is carried by the pulmonary veins to the left auricle, 

 from there to the left ventricle, and thence, by means of the aorta and its 

 branches, to the system at large. There is, therefore, in birds a perfect 

 double circulation a pulmonary and a systemic circulation (Fig. 191). 



FIG. 190. HEART OF THE CROCODILE. 



(Perrier.) 



a a, venae cavae : b, right anricle ; r. right ventricle ; 

 <L left ventricle : e, pulmonary veins : /, left auricle ; It hf. 

 aortae : i. second arch of the aortae. in which arterial and 

 venous blood is mixed ; g, pulmonary arteries. 



FIG. 191. DIAGRAM OF THE CIRCULATION 

 IN BIRDS AND MAMMALS. (Carpenter.) 



A, heart ; B, vena cava ; C, right auricle ; D, right 

 ventricle ; E, pulmonary arte^ 1 - ; F, pnlmonic capillaries ; 

 G, left auricle; H, left ventricle ; i, aorta; J, systemic 

 capillaries. 



The portal system is also present in birds, and, as in the lower verte- 

 brates, receives branches from the kidneys and from the lower limbs. 



The circulation in mammals, of which man may be taken as a type, 

 deserves to be treated at somewhat greater length ;than the circulation in 

 inferior organisms. As in other vertebrates, with the exception of the 

 openings, of the larger lymphatics, the blood is contained in a com- 

 pletely closed S3 r stem of vessels, whose course and general arrangement 

 we will first trace in outline. 



The circulation is carried on through a S3 T stem of tubes of different 

 functions and different properties. These are : (1) the central organ, the 

 heart, a hollow muscle, which serves as both a pump and a reservoir, 

 divided into four cavities, two auricles or receivers of blood, and two 

 ventricles or pumps (Fig. 192) ; (2) the arteries, a system of muscular 



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