448 THE RABBIT. PHYLUM CHORDATA 



thymus. It has four chambers, two auricles and two ventricles. 

 Three vense cavse corresponding to those of the frog open directly 

 into the right auricle (Fig. 354), and two pulmonary veins lead 

 by a common opening into the left auricle. The opening from the 

 right auricle into the right ventricle is guarded by a threefold 

 tricuspid valve fastened to the ventricular wall by chordae 

 tendineae, and a similar twofold mitral valve guards the opening 

 between the chambers of the left side. The two sides do not 

 communicate with one another. From the front end of the right 

 ventricle arises the pulmonary artery, and from the front of the 

 left ventricle, above the pulmonary artery, arises the single aortic 

 arch. The opening of each of these vessels is provided with three 

 semilunar valves. The pulmonary artery divides to supply the 

 two lungs, and the arteries to the head and arms arise from the 

 arch of the aorta, which afterwards supplies the trunk. In the 

 beating of the heart, the auricles contract simultaneously, and 

 the ventricles follow immediately afterwards ; then after a short 

 pause the auricles start another contraction. The venous blood 

 which reaches the right auricle from the capillaries of the body 

 is driven by the auricular contraction into the right ventricle 

 and thence in turn through the pulmonary artery to the lungs. 

 Returning oxygenated to the left auricle it is driven into the left 

 ventricle, and thence through the aorta to all parts of the body. 

 There is thus a double circulation, as in the frog, but the separa- 

 tion of the ventricles and connection of the pulmonary artery 

 with one of them and the aorta with the other ensures that there 

 is no mixing of oxygenated and deoxygenated blood. 



REGULATION OF THE CIRCULATION 



The supply of blood which an organ receives depends on two 

 factors : (i) the width of the small blood vessels in the organ, 

 (2) the pressure under which the blood is flowing. When an organ 

 such as a muscle is active, its small vessels are caused to dilate 

 by the presence of carbonic acid (p. 18) and other products of 

 the activity of its tissues, and also by the action of the nervous 

 system. Now any dilatation of blood vessels, by enlarging the bed 

 of the blood stream, tends to lower the general blood pressure, and 

 thus both to diminish the local effect of enlarging the vessels, and 

 also to have injurious results in other organs. These tendencies, 

 however, if they be on a sufficient scale, are counteracted through 



