134 ANATOMY AND PHYSIOLOGY. 



Superior Pulm. Pulmonary 



vena cava. art. Aorta, artery. 



" ' Pulmonary veins. 



Pulmonary veins. ^..TM^^jJ^y^^J^ .- Left auricle. 

 Eight auricle. ^^fflW^ **** ^ 

 Tricuspid valve. '"' 

 Inferior vena. cava. -'- W\3J| I HV/- Left ventricle. 



Right ventricle. , . 



Partition. Aorta. 



by a strong vertical partition, that divides the entire heart into two 

 halves, which are almost exactly similar to each other, excepting 

 in the greater thickness of the walls on the left side ; and each of 

 these halves (which do not communicate with one another) is again 

 subdivided by a transverse partition, into two cavities, of which the 

 upper one is termed the auricle^ and the lower the ventricle. Thus 

 we have the right and left auricles, and the right and left ventricles. 

 Each communicates with its corresponding ventricle, by an aperture 

 in the transverse partition, which is guarded by a valve. The walls 

 of the ventricles are much thicker than those of the auricles ; and 

 for this evident reason, that the ventricles have to propel the 

 blood, by their contraction, through a system of remote vessels ; 

 whilst the auricles have only to transmit the fluid that has been 

 poured into them by the veins, into the ventricles, which dilate 

 themselves to receive it. The comparative thickness of the walls 

 of the left and right ventricles is explainable on the same principle ; 

 for the left ventricle has to send the blood, by its contractile 

 power, through the remotest parts of the body ; whilst the right 

 has only to transmit it through the lungs, which, being much 

 nearer, require a far less amount of force for the circulation of the 

 blood through them. 



We are thus brought, then, to consider a second important depart- 

 ment of our science, the circulation of the blood ; but, first, it is 

 necessary to inquire into the nature of the blood itself. 



With the appearance of blood, as it occurs in the higher classes 

 of animals, every one is familiar. When drawn from one of the 

 vessels which immediately receive it from the heart, and which are 

 called arteries, it is of a bright scarlet color ; but when taken, as it 

 usually is in the common operation of bleeding, from a vein, it is 



