THE CIRCULATION. 175 



with air, rough surfaces and rest, and retarded by cold, neutral 

 salts in excess, exclusion of air, etc. (Brubaker). 



Arterial Blood contains more oxygen and less carbon dioxide 

 than venous ; is scarlet in color, from the presence of oxy-hsemo- 

 globin. Venous blood is purple in color. 



The blood of \ht portal vein is rich in water, albuminous mat- 

 ter and sugar, and the hepatic is rich in corpuscles and sugar. 



CIRCULATION OF THE BLOOD. 



The blood may be traced from the left ventricle, through the 

 aorta, to the capillaries, where it passes into the veins, being re- 

 turned to the right auricle by the anterior and posterior cavas. 

 From there it passes to the right ventricle, through the pulmonary 

 artery, to the lungs, where it is oxygenated. It is then returned 

 to the left auricle by the pulmonary veins, and from there to the 

 ventricle. By this it is seen that the heart is the central organ 

 of the circulation, and propels the blood by alternate contractions 

 and dilatations. The contraction is known as the systole, the dila- 

 tation as the diastole. 



The phenomena attending the systole are a hardening, shorten- 

 ing, a movement of the apex downward, and a twisting of the 

 heart from left to right. 



If the heart's cycle is divided into ten tenths, during the first 

 four-tenths the ventricles contract simultaneously and the auricles 

 diXQ passive ; in the second four-tenths the auricles dilate and the 

 ventricles 2iXQ passive ; in the last two-tenths the auricles contract 

 and the ventricles dilate. 



The sounds of the heart are known as the first and second,^ 

 with an interval of silence after the latter. The first sound occu- 

 pies four- tenths, the second three -tenths, the interval of silence 

 three-tenths. 



The fif'st sound is longer, louder, lower in pitch and more 

 booming than the second, and is caused by (i) the closing of the 

 auriculo-ventricular valves; (2) the apex striking against the 

 chest ; (3) the contraction of the muscular fibres ; (4) the sound 

 of the blood passing through the interior. 



The second sound \<> shorter, higher in pitch, and clicking, and 

 is caused by the closure of the mitral and tricuspid valves. 



The force of the heart, or vis a tergo, is more than sufficient to 

 carry the blood through the system. 



