THE HEART AND CIRCULATION OF BLOOD. 311 



The Heart and Circulation of the Blood. 



The circulatory apparatus consists of a central muscular heart 

 and three sets of vessels. 



Two fluids are found in connection with the circulatory ap- 

 paratus — viz., blood and lymph. Blood is of two kinds, arterial 

 and venous. The former is pure and red, the latter impure 

 and dark. Lymph consists of a white to pale yellow fluid, 

 and contains lymph - corpuscles. Intimately connected with 

 this lymph is chyle. Chyle is a fluid that comes from 

 the walls of the alimentary canal ; it is lymph charged 

 with nutritive matter. Lymph is blood minus the red blood- 

 corpuscles. Chyle is lymph plus fat, &c. We can detect 

 three sets of vessels, as follows : (i) extend from the heart 

 to all parts of the body, and which contain red blood = 

 arteries, (ii) pass from all parts of the body to the heart, 

 and which contain dark blood = veins ; (iii) pass from the 

 majority of organs and walls of intestines to one of the veins 

 = lymphatic vessels. 



The heart is a muscular box, by means of which the 

 blood is circulated or pumped throughout tlie whole system. 

 The muscle composing the heart is peculiar in structure ; 

 it is known as cardiac muscle, and is cubical in form and 

 striated, and of course involuntary in action. The heart 

 lies in a sac closely applied to its walls, the pericardium, 

 and is bathed by a fluid, the pericardial fluid. There are 

 four distinct chambers ; the two upper are the auricles (fig. 162, 

 LA and RA), the two lower the ventricles (F). The auricles 

 receive the convergent tubes, the veins ; the ventricles give 

 origin to the divergent vessels, the arteries. The right and 

 left sides of the heart are quite separate, but the two cavities 

 of each half are in connection, the openings being guarded by 

 valves. The opening from the right auricle into the right 

 ventricle, the " auriculo-ventricular " opening, is closed by the 

 tricuspid valves, which consist of three muscular flaps; the 



