THE VASCULAR SYSTEM. 393 



or leucocytes, of much physiological importance, e.g., as 

 bearers of food-particles from one part of the body to another. 



The heart or central organ of the circulation receives 

 blood from veins, and drives it forth through arteries. Its 

 activity is the chief cause of 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 all Fishes except Teleost- 

 eans and in Amphibians, the blood passes from the ven- 

 tricle 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 Vertebrates higher than Amphibians these parts have 

 virtually disappeared. 



In Cyclostomata, and in all Fishes except Dipnoi, the 

 heart has one auricle and one ventricle. In these animals 

 the heart 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 has two auricles, and there is a slight 

 partition in the ventricle. 



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

 The right auricle always 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. 



