THE CIRCULATION OP THE BLOOD. 



247 



ventricle characterized by a spongy arrangement of the muscle- 

 flbers of its walls. 



In the reptiles the division between the auricles is complete, 

 and there is one ventricle which shows imperfect subdivisions. 



/>r.e 



Pig 213. 



Pig. 212.— The frog's heart, seen from the front, the aortic arches of the left side hav- 

 ing been removed. (1 x 4.) ca, carotid; c. gl, carotid gland; oo, aorta; o-w', ri^ht 

 auricle; au", left auricle; pr. c, vena cava superior; pt. c, vena cava inferior; 

 p. cu, pulmo-cutaneous trunk; tr, truncus arteriosus: v, ventricle (Howes). 



Pig. 213. — The same, seen from behind, the sinus venosus having been opened up to 

 show the sinu-auricular valves. (1 x 4.) p. -y, pulmonary vein; *, % sinus veno- 

 sus; va", sinu-auricular valve. Other lettering as in Fig. 212 (Howes). 



In the crocodile, however, the heart consists of four per- 

 fectly divided chambers. Of the two aortic arches, one arises 

 together with the pulmonary artery from the right ventricle, 

 and, as it crosses over, the left communicates with it by a small 

 opening, so that, although the arterial and the venous blood 

 are completely separated in the heart, they intermingle outside 

 of this organ. 



In birds the circulatory system is substantially the same as 

 in mammals ; but in all vertebrate forms below birds the blood 

 distributed to the tissues is imperfectly oxidized or is partially 

 venous. 



As a result of the entire vascular arrangements in the frog, 

 etc., the least oxidized blood passes to the lungs, and the most 

 aerated to the head and anterior parts of the animal. 



Whatever ground for differences of opinion there may be 

 as to the extent to which the phenomena we have as yet been 

 describing are mechanical in their nature, all are agreed that 



