THE LUNGS. 219 



oxygenated. After the blood has been revivified, or made 

 arterial, it is brought back to the left auricle by the pul- 

 monary veins. Thus much of the motion of the blood is 

 termed the pulmonic circulation. The peculiarity of the 

 pulmonic circulation is that arteries carry venous blood, and 

 veins are the means of conveyance for arterial blood. 



The greater, or general circulation. — We have traced the 

 blood into the left auricle, into which it is poured upon 

 the dilatation of that cavity. The left ventricle dilating, 

 and the left auricle contracting the blood, is urged into the 

 lower cavity upon the left side. From the left ventricle it 

 is driven into the aorta ; thence into the right and left 

 aorta ; so into the various vessels and their capillaries, to 

 nourish the different parts of the body. Having performed 

 this office, it is returned from the capillaries by the veins, 

 and ultimately by the two cavas into the right ventricle of 

 the heart. It appears that the contraction and dilatation 

 of the two ventricles take place at the same instant ; and 

 that at the same time that the blood is driven fi^om the left 

 ventricle into the aorta, the right ventricle impels it into 

 the pulmonary artery : and as the period at which the left 

 auricle receives the pulmonary blood is the same with that 

 in which the right auricle receives the blood of the cavas ; 

 so it will be evident that the relaxation of the ventricles 

 and the contraction of the auricles are simultaneous. These 

 alternate contractions and dilatations of the heart are called 

 its systole and diastole. The contraction of the ventricles, 

 or its systole, pressing the blood into the pulmonary artery 

 and the aorta ; it follows, therefore, that when the heart 

 is in a state of contraction, or systole, it is at this period 

 we feel what is termed their pulsation. Thus it will appear, 

 that the two nominal circulations form but one direct round 

 of transmission through the body. The nerves of the heart 

 are principally the parvagum and sympathetic. 



THE LUNGS. 



The cavity of the chest is occupied by two soft and 

 slightly elastic masses, called lungs {Plate VIII. B, C). In 

 a state of distention they completely fill the parieties of the 

 thorax, to which their figure is exactly adapted (see Plate 

 VIII.). The lungs have a distinct division into a right and 



