68 PHYSIOLOGY. 



and thus, at the same time, relieves the distended right ventricle, and 

 conveys to the left the due stimulus to its action. In conclusion, it 

 may be stated, that the post-mortem appearances are always the 

 same in asphyxiated individuals, no matter what may have been the 

 mode of death. 



CIRCULATION. 



By this term is understood that function by which the nutritive 

 fluid is conveyed to every part of the body through appropriate 

 channels. This distribution is spoken of under the general title of 

 The circulation of the blood; the organs and canals by means of 

 which and through which it is accomplished constitute the vascular 

 system. 



The greater circulation was discovered in the higher animals by 

 William Harvey in 1619, and although it cannot be asserted to be 

 an universal character of all animals, yet at every advance of obser- 

 vation, new traces of vessels are discovered in the most simple 

 beings. 



There is a circulation in plants as well as in animals, and its ob- 

 ject is the same. There is no central organ, however ; the elaborated 

 sap is distributed partly by vis a tergo, partly by capillary action, 

 and the influence of light and air upon the leaves, but mainly by 

 the affinity between the elaborated sap and the tissues of the plant. 



In the higher orders of animals, there are two distinct circulations, 

 with a heart for each. These two circulations are entirely separate 

 and distinct from each other in the perfect adult, as are also the 

 hearts belonging to each, being merely brought together for economy 

 of material and space. They are called the greater and the lesser ; 

 or the systemic and pulmonic. 



The course of the blood through these two circuits, may be likened 

 to the figure 8 ; and the heart is placed at the junction of these. 

 Each system has its own set of arteries or efl^erent trunks, and veins 

 or afl?erent trunks ; these communicate at their central extremity 

 by the heart; and at their peripheral extremity by the capillary ves- 

 sels, by which are meant the minutest ramifications of the two sys- 

 tems inosculating into a plexus. 



The route of the circulation is as follows : the venous blood is 

 collected in the right auricle, thence it passes through the tricuspid 

 orifice into the right ventricle ; thence it is distributed to the lungs 

 through the pulmonary artery to be aerated, after which it is collected 

 by the pulmonary veins and carried to the left auricle, from which 

 it passes through the mitral orifice into the left ventricle, and from 

 thence through the aorta to the system. 



The chief impelling power of the circulation, is the rhythmic mo- 

 tion of the heart. This organ is endowed in a remarkable degree 

 with the property o^ irritability or contractility ; by which is meant 



