164 



CIRCULATION OF THE BLOOD 



under normal circumstances their form depends in the main upon the degree 

 to which they are filled. In systole when, as Harvey put it, " the heart makes 

 tense all of its fibers/ 7 the ventricles whether empty or filled have a perfectly 

 definite form, which is entirely independent of the diastolic form. Hence if 

 the heart is lengthened in any one of its. diameters during diastole, it is short- 

 ened in this diameter during systole. 



In the living body and in the unopened chest the heart lies in the peri- 

 cardium and is covered for the most part by the lungs. It is suspended upon 



FIG. 53 Casts of the ventricular cavities of the ox heart in rigor mortis, after Worm-Muller 

 and Sandborg. A, cavity of the right ventricle; B, of the left. Two-thirds natural size. 



the great arteries and so far as the pericardium will permit, is movable in dif- 

 ferent directions. 



If one observes the heart of a mammal in the usual supine position of the 

 animal in experimentation, the diastolic heart is flattened somewhat in the 

 anterior-posterior direction, while its transverse diameter is increased. Under 

 such circumstances one finds that the long axis and the transverse diameter 

 shorten in systole, while the sagittal axis becomes longer. 



In the natural position of the body the heart is supported for the most part 

 by the lungs ; these are to be looked upon as air cushions which influence the 

 form of the diastolic heart only to a slight extent. In the natural position of 

 the animal therefore the base of the heart must be more circular than in the 

 supine position with the chest open. By means of needles stuck through the 

 chest wall into different parts of the heart, so that their ends gave by their 



