CH. xix.] THE HEAHT. 22Q 



Fluid always flows in the direction of pressure ; it could no more 

 flow from a place where the pressure is low to where it is high 

 than a river could flow uphill. This difference of pressure is 

 produced in the first instance by the contraction of the heart, but 

 we shall find in our study of the vessels that some of this 

 pressure is stored up in the elastic arterial walls, and keeps up 

 the circulation during the periods of rest of the heart. 



Before passing on to consider the physiology of heart and 

 vessels at greater length, let us take a few types of the circulatory 

 system from different parts of the animal kingdom. 



In worms, and in the lowest vertebrate Amphioxus, the cir- 



Fig. 2.}2. Tlie heart of a frog (Rana esculenta) from the front, r, ventricle ; Ad, right 

 auricle; As, left auricle; /', bulbus arteriosus, dividing into right and left aortee. 



(Ecker.) 



dilatory system is almost as simple as in the schema just 

 described ; the heart is a long contractile tube provided with 

 valves, which contracts and presses the blood forwards into the 

 aorta at its anterior end ; this divides into arteries for the supply 

 of the body ; the blood passes through these to capillaries, and ia 

 collected by veins which converge to one or two main trunks that 

 enter the heart at its posterior end. 



In fishes, the heart is a little more complicated ; it 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 



