230 



THE CIRCULATION OF THE BLOOD 



[CH. XX. 



and lymph. The heart pumps the circulating fluid along a system 

 of vessels which distribute it over the body ; there are no capillaries, 

 and the hsemolymph is discharged into the tissue spaces ; it is thence 

 drained into channels which convey it to the gills, and after it is 

 aerated there in a set of irregular vessels, it is returned to the peri- 

 cardium. It is sucked from the pericardium into the heart during 

 diastole, through five small orifices in the cardiac wall; during 

 systole these are closed by valves. In these animals the rate of flow 

 of hcemolymph is necessarily slow. 



In worms, the circulatory system is almost as simple as in the 



schema just described ; the heart is 

 a long contractile tube provided 

 with valves, which contracts peri- 

 staltically and presses the blood 

 forwards into the aorta at its an- 

 terior end ; this divides into arteries 

 for the supply of the body; the 

 blood passes through these to capil- 

 laries, and is collected by veins 

 which converge to one or two main 

 trunks that enter the heart at its 

 posterior end. 



In fishes, the heart is divided 

 into a number of chambers placed 

 in single file, one in front of the 

 other; the most posterior which 

 receives the veins is called the 

 sinus venosus; this contracts and 

 forces the blood into the next 

 chamber, called the auricle; this 

 forces the blood into the next 

 cavity, that of the ventricle, and 

 last of all is the aortic bulb. From the bulb, branches pass to the 

 gills, where they break up into capillaries, and the blood is aerated : 

 it then once more enters larger vessels which unite to form the 

 dorsal aorta, whence the blood is distributed by arteries to all 

 parts of the body ; here it passes into the systemic capillaries, then 

 into the veins which enter the sinus (whence we started) by a 

 few large trunks. 



Taking the frog as an instance of an amphibian, we find the 

 heart more complex, and the simple peristaltic action of the heart 

 muscle as we have described it in the hearts of worm and fish, 

 becomes correspondingly modified. There is only one ventricle, but 

 there are two auricles, right and left. 



The ventricle contains mixed blood, since it receives arterial 



Fro. 212. The heart of a frog (Rana esculenla) 

 from the front. V, ventricle ; Ad, right 

 auricle ; As, left auricle ; B, bulbus arteri- 

 osus, dividing into right and left aortse. 

 (Bcker.) 



