THE HEART. 



265 



3d. The capillaries ; a network of minute inosculating tubules, 

 which are interwoven with the substance of the tissues, and which 

 bring the blood into intimate contact with the cells and fibres of 

 which they are composed ; and 4th. The veins ; a set of converg- 

 ing vessels, destined to collect the blood from the capillaries, and 

 return it to the heart. In each of these four different parts of the 

 circulatory apparatus, the movement of the blood is peculiar and 

 dependent on special conditions. It will therefore require to be 

 studied in each one of them separately. 



Fig 



THE HEART. 



The structure of the heart, and of the large vessels connected 

 with it, varies considerably in different classes of animals, owing to 

 the different arrangement of the respiratory organs. For the respi- 

 ratory apparatus being one of the most important in the body, and 

 the one most closely connected 

 by anatomical relations with 

 the organs of circulation, the 

 latter are necessarily modified 

 in structure to correspond with 

 the former. In fish, for exam- 

 ple (Fig. 76), the heart is an 

 organ consisting of two princi- 

 pal cavities ; an auricle (a) into 

 which the blood is received from 

 the central extremity of the 

 vena cava, and a ventricle (b) 

 into which the blood is driven 

 by the contraction of the auricle. 

 The ventricle is considerably 

 larger and more powerful than 

 the auricle, and by its contrac- 

 tion drives the blood into the 

 main artery supplying the gills. 

 In the gills (cc) the blood is 



. ,. , . . CIRCULATION OF FISH. a. Anncle. r>. 



arteriallZed ; after Which it IS Ventricle, cc. Gills, d. Aorta, ee. Venae cava. 



collected by the branchial veins. 



These veins unite upon the median line to form the aorta (d) by 



which the blood is finally distributed throughout the frame. In 



