228 



PHYSIOLOGY. 



The external fibrous, or parietal, layer is a strong, inelastic 

 membrane which embraces the origin of the great blood-vessels at 

 the base of the heart. 



These two layers unite to make a close sac. Between the pari- 

 etal and visceral layers is the pericardial liquor, which permits the 

 two layers to slide on each other without friction. The elastic 

 fibers in the parietal layer permit of its following very closely the 



changing form of the heart. 



Fig. 67. Course of the Ventricular Muscular Fibers. (LANDOIS.) 



A, On the anterior surface. B, View of the apex with the vortex. G, Course 

 of the fibers within the ventricular wall. D, Fibers passing into a papillary 

 muscle, P. 



The Auricles. In examining each half of the heart it is easy 

 to recognize that the auricle, on account of the thinness and the 

 weakness of its muscular walls, can scarcely be the important part 

 of that organ. In laying bare the heart of an animal while artificial 

 respiration is maintained, it is seen that the action of the auricle is 

 very weak as compared with that of the ventricle. A manometer 

 introduced into the auricular cavity at the moment when it con- 

 tracts marks a pressure that is five or six times less than that 

 obtained in the corresponding ventricular cavity under the same 

 conditions. 



