538 



GENERAL CHARACTERS OF REPTILES. 



ao c c a 



od 



behind the heart, goes to empty itself into the descending aorta (ao). 

 It follows from this, that at each contrac- 

 tion of the heart, one portion of the 

 venous blood is carried to the lungs, and 

 another portion goes to be mixed with 

 arterial blood ; but this mixture is not 

 carried on in the first part of the interior 

 of the aorta, but h.elow the origin of the 

 branches (c c) which this vessel sends to 

 the head and anterior part of the body, 

 so that these parts receive pure arterial 

 blood ; whilst all those parts, whose 

 arteries arise below the point of junc- 

 tion of the aorta, with the vessel arising 

 from the right ventricle, only receive 

 a mixture of red and dark blood. 

 It was believed until recently, that 

 among other animals of this class (the 

 Batrachians, or Frog tribe), there only 

 existed a single auricle; but this has 

 been shown to be otherwise. With 

 respect to the mode of distribution of 

 the arteries among .Reptiles, we shall 

 only add, that there exist two or more 



aortic arches curving to the right and to the left, and shortly 

 uniting to constitute one trunk (Fig. 314). 



479. The respiration of Reptiles is not active ; the greater 

 number of these animals consume but little oxygen, and can be 

 deprived of it for a long time without being suffocated. More- 

 over, temperature has very great influence upon this phenomenon ; 

 and in warm weather, the necessity of respiration is felt much 

 more than in winter. A Frog, for example, which has been 

 deprived of air, perishes in summer in less than two hours, whilst 

 in winter it may continue to live for several days. In some 

 Reptiles, as we shall shortly see, gills exist in the early period 

 of life ; but the lungs are soon developed, and the gills usually 

 then wither and disappear ; so that the same animal h as at first 



FIG. 313 HEART AND LARGE 

 VESSELS OF CROCODILE. 



vv, systemic veins, termi- 

 nating in the right auricle, 

 od ; vt, the two ventricles, 

 separated by an internal par- 

 tition ; ap, the two pulmo- 

 nary arteries, conveying blood 

 from the right ventricle to 

 the lungs ; a, trunk from the 

 same ventricle, going to join 

 the descending aorta ; vp, 

 pulmonary veins, emptying 

 into left auricle, og; ao, aorta, 

 arising from left ventricle ; 

 c, c, vessels proceeding to 

 the head. 



