28 



CIRCULATION OF THE BLOOD. 



FIG. 1. 



Describe the course of the blood in 

 circulation, beginning with its 

 entrance into the left auricle. 

 Into the left auricle from pul- 

 monary veins, thence passing the 

 open mitral valve into the left ven- 

 tricle (Fig. 1). Upon contraction 

 of the ventricle the mitral valve is 

 closed and the aortic valve thrown 

 open, so that the blood is thrown 

 into the aorta, and thence through 

 the systemic arterial circulation into 

 capillaries and on into veins, the 

 systemic veins finally joining to fill 

 the venae cavse, and from them the 

 right auricle. From the right auricle 

 past the tricuspid valve into the 

 right ventricle, whence it is thrown 

 through the pulmonary artery 

 (guarded by the pulmonary semi- 

 lunar valve) into the pulmonary 

 capillaries, and thence into the pul- 

 monary veins, whence it started. 

 Thus we have in reality two cir- 

 culations, the systemic and pul- 

 monary. 



Describe the heart. 



The heart is a muscular organ 

 situated in the thorax, where it lies 

 between the lungs within the peri- 

 cardial sac, and rests upon the dia- 

 phragm somewhat to the left of the 

 mid-line of the body. It is con- 

 ical in form, and is so suspended 

 by the great vessels that the apex 



?" oints to the left and downward, 

 ts size is about that of the closed 

 fist (weight, in adults, about 10 

 ounces). 



Diagram of the Circulation: 1, heart; 2, lungs; 3, head 

 and upper extremities; 4, spleen; 5, intestine; 6, kid- 

 ney ; 7, lower extremities ; 8, liver. 



