188 BIOLOGY 



in every direction, so that when they contract (systole) the 

 heart is diminished in size and the blood that is in the cavities 

 is squeezed out; when they relax (diastole) the heart expands 

 again and the blood flows into it. Figure 92 shows a diagram 

 of the heart structure, cut open so as to show the interior of 

 the cavities. At the anterior end are two cavities, the right 

 and the left auricles, ra and la; the right, which receives blood 

 from the body, being much larger than the left, which receives 

 blood from the lungs. These two chambers are separated 

 by a partition. At the lower side of the auricles each opens 

 into the ventricle, v, the third and largest chamber below. 

 The openings between the auricles and ventricle (shown by the 

 arrows in Fig. 102), are guarded by valves, which are flaps of 

 membrane, so situated that they allow blood to flow readily 

 from the auricle into the ventricle, but close up at once if the 

 blood starts to flow back into the auricle, as it would do when 

 the ventricle contracts, did not these valves block the passage. 

 The ventricle is a large chamber, partly divided by partitions. 

 Leading out of it in front is a large blood vessel. This 

 extends forward, on the ventral side of the heart, and 

 at the anterior end of the heart it divides into two arteries, 

 one turning to the right and one to the left. The large 

 vessel is called the bulbus arteriosus, ba, and its two 

 branches are the aortae, ad. This bulbus receives the blood which 

 is forced out of the heart when it contracts. Within it, and 

 at the beginning of the aorta?, are valves which control the 

 flow of the blood, as will be described on a later page. On the 

 dorsal side of the heart is a large thin-walled chamber, the ve- 

 nus sinus (Fig. 92 B, vs), into which open the veins that bring 

 the blood back from the body. This sinus opens into the right 

 auricle, which thus receives all the blood which flows back to 

 the heart from all parts of the body, except the lungs. The 

 blood from the lungs empties into the left auricle by two small 

 veins, one from each lung; Fig. 92 pv. 

 The blood vessels ramify all over the body in a very complex 



