VASCULAR SYSTEM. 457 



circular. Moreover, the full-grown red corpuscles of 

 Mammals have no visible nuclei. The blood fluid also 

 contains uncoloured nucleated amoeboid cells, the white 

 corpuscles or leucocytes, of much physiological importance. 

 Some of them, specialised as phagocytes, form " a body- 

 guard," 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 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. The partition in the ventricle secures that 

 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. 



