CARDIAC NERVES 185 



The course from the right ventricle through the lungs and back 

 to the left auricle is called the pulmonary circulation (Figs. 125 and 

 1 60). 



The course from the left ventricle through the entire body or 

 " system" and back to the right auricle is called the systemic 

 circulation or main circulation (Fig. 126). 



Important notes. Pure blood is carried from the heart through the 

 systemic arteries to all tissues in the body to nourish them. This blood is 

 called arterial blood; it is bright red in color. The term pure blood and 

 arterial blood are used to signify one and the same thing. 



Impure blood from the tissues of the body is returned to the heart by the 

 systemic veins. It is called venous blood; it is purple-red or blue in color and 

 contains waste matters. The terms impure blood and venous blood are used to 

 signify one and the same thing. 



The venous blood from the body is poured into the right side of the heart, 

 from which the pulmonary artery conveys it to the lungs. Consequently the 

 pulmonary artery is unlike others, because it carries venous blood from the 

 heart; and the pulmonary veins are unlike others because they carry arterial 

 blood to the heart. 



Innervation of the heart. As already mentioned, p. 176, the 

 heart muscle or myocardium has a structure peculiar to itself. 

 Anatomically its fibers differ from those of other muscles in size 

 and arrangement; physiologically it is unlike any other striped 

 muscle in the body, being involuntary although striped. Cardiac 

 muscle responds to the stimulus brought by two sets of nerves, 

 called accelerator and inhibitor nerves. By the accelerator nerves 

 the rapidity of the heart-beat is increased; by the inhibitors it is 

 slowed. These nerves are branches of the pneumogastric or vagus 

 nerve. A long unsettled question is, whether in the absence of 

 these, the heart would still beat under the chemical stimulus alone 

 of tissue change. Years of experimentation and observation 

 incline scientists to believe that this is possible. 



The heart muscle of cold-blooded animals performs rhythmic contractions 

 for several hours after removal from the body; to insure this action it is only 

 necessary to keep it moistened in a mixture of calcium, potassium, and sodium 

 salts in solution. The chemical action between this fluid and that within the 

 cells is sufficient to produce contractions, following each other in rhythmic 

 order. 



So in life, an interchange of chemical products between lymph and cell 

 contents may furnish a stimulus which keeps the heart in action, the 



