ANATOMY OP THE HEART. 161 



of the ventricles from the distal end of the primitive cardiac 

 tube. 



The fibers of the auricles run transversely, beginning and end- 

 ing in the fibrous tissue which separates the auricles from the 

 ventricles. The musculature of the ventricles is somewhat hard- 

 er to trace. There are layers that run transversely around the 

 ventricles, and also layers which describe more or less of a spiral 

 course from the base of the ventricles to the apex and then are 

 reflected back in transverse layers, until they finally end in the 

 papillary muscles, which are connected with fibrinous threads. 



Fig. 16. A generalized view of the vertebrate heart (Keith) showing: a, 

 the sinus venosus ; b.c., the auricle ; 33, the auriculo-ventricular orifice and 

 valves; d, the ventricle; e, the beginning of the aorta with the semilunar 

 valves at 5. The valves between e and / do not exist in the heart of man. 

 (Prom Howell's Physiology.) 



the chorda tendinea?, to the edge of the auriculo-ventricular 

 valves. 



When the ventricles contract, this arrangement of muscular 

 fibers causes the apex and the base of the heart to approach one 

 another, and the transverse section is changed from an ellipse to 

 a circle. The base of the heart, hung as it is to the large vessels 

 in the thorax, appears to be fixed, and one would expect that the 

 apex is the part which moves up and down. This is not the case, 



