454 STRUCTURE OF VERTEBRATA. 



and by attacking and destroying micro-organisms within 

 the body. 



The heart receives blood from veins, and drives it forth 

 through arteries. Its contractions in great part cause the 

 inequality of pressure which makes the blood flow. It lies 

 in a special part of the body cavity known as the pericardium, 

 and develops from a single blood vessel in Cyclostomata, 

 Fishes, and Amphibians, from a pair in Reptiles, Birds, and 

 Mammals. 



The receiving region of the heart is formed by an auricle 

 or by two auricles, thence the blood passes into the muscular 

 ventricle or ventricles, and is driven outwards. Except in 

 adult Birds and Mammals the veins from the body enter the 

 auricle (or the right auricle if there are two) by a porch 

 known as the sinus venosus. In Fishes (except Teleosteans) 

 and in Amphibians, the blood passes from the ventricle into 

 a valved conus arteriosus which seems to be a continuation 

 of the ventricle. In Teleosteans there is a superficially 

 similar structure, but without valves and non-contractile, 

 and apparently developed from the aorta, not from the 

 ventricle ; it is called the bulbus arteriosus, and may occur 

 along with the conus arteriosus in other Fishes. In Verte- 

 brates higher than Amphibians the conus is, to say the 

 least, less distinct. 



In Cyclostomata, and in all Fishes except Dipnoi, the heart has one 

 auricle and one ventricle, and contains only impure blood, which it 

 receives from the body and drives to the gills, whence purified it flows 

 to the body. 



In Dipnoi, the heart is incipiently three chambered. 



In Amphibians, the heart has two auricles and a ventricle. The right 

 auricle alw T ays receives venous or impure blood from the body, the left 

 always receives arterial or pure blood from the lungs. The single 

 ventricle of the Amphibian heart drives the blood to the body and to 

 the lungs. 



In all Reptiles, except Crocodilia, the heart has two auricles and an 

 incompletely divided ventricle. By means of the partition in the 

 ventricle much of the venous blood is sent to the lungs ; indeed the 

 heart, though possessing only three chambers, works almost as if it 

 had four. 



In Crocodilia, there are two auricles and two ventricles. But the 

 dorsal aorta, which supplies the posterior parts of the body, is formed 

 from the union of two aortic arches, one from each ventricle. Therefore 

 it contains mixed blood. 



In Birds and Mammals, the heart has two auricles and two ventricles, 



