BIOLOGY. 



[BOOK ir. 



divided into two parts, an auricle and a ventricle. The first part 

 receives the venous blood coming from the rest of the body, and 

 impels it into the second cavity, whence it is sent forth again by 

 a system of vessels into the respiratory or branchial organs. In 

 the branchial folds the blood traverses a system of fine capillary 

 vessels ; it is there charged with oxygen, and is afterwards poured 

 into the general circulation to return anew to the heart. (Figures 

 19 and 20.) 



When lungs take the place of branchiae the heart commences 

 to divide longitudinally into four 

 cavities more or less incompletely 

 separated. There are, first of all, 

 in the auricles a reticulated tissue ; 

 in the ventricle protuberances, mus- 

 cular projections (lepidosiren). In 

 the reptiles there are three cardi- 

 acal cavities ; two auricles, the one 

 receiving the venous blood of the 

 body, the other the blood returning 

 from the respiratory organs. The 

 separation of the ventricle into two 

 cavities is indicated only by a re- 

 fciculated tissue. Sometimes, how- 

 ever, these two reticulated halves 

 do not contract simultaneously (tor- 

 toises). Here still, as in the inferior 

 crustaceans and the arachnida, the 

 totality of the blood is not constrained 

 to traverse the respiratory surfaces before returning to the 

 heart. 



In birds the longitudinal separation of the heart is complete ; 

 there are two auricles, two ventricles. The blood vivified, 

 oxygenised, returning from the lungs > the arterial blood, is no 

 longer mingled in the heart with the blood which has served for 

 nutrition, the venous blood, black and charged with carbonic 



\ 



FIG. 21. 



Heart airl large vessels of Sctlaman- 

 dra maculosa. The first arc of the 

 aorta, c, in direct continual ion with 

 the carotid ; w, x, y, z, apparatus of 

 the hyoid bone j c, carotidian 

 glands. 



