THE HEART BEAT. 



531 



logically the four-chambered heart is developed from a simple 

 tube, and this origin is indicated in the adult by the fact that the 

 musculature of the two auricles is in large part common to both 

 chambers, — that is, surrounds them as though they were a single 

 chamber, — and the same is true of the ventricles. In the auricles 

 there is a superficial layer of fibers which runs transversely and en- 

 circles both auricles. The simultaneous contraction of the two 

 chambers would seem to be insured by this arrangement alone. 

 In addition, each auricle possesses a more or less independent 

 system of fibers, whose course is at right angles to that of the 



Fig. 221. — Schema to show the course of the superficial and deep fibers of the bulbo- 

 Bpiral and sinospiral systems. The heart is viewed from the dorsal side. BS, superficial butbo- 

 spiral system; BS', deep bulbospiral system; SS, superficial sinospiral system; SS .deep smo- 

 spiral system; C, circular fibers round the conus; C, circular fibers round the base of the aorta 

 and the left ostium; LRV, longitudinal bundle of right ventricle, from membranous septum to 

 right ventricle; IV, interventricular or interpapillary layer (Mall). 



preceding layer. These fibers may be considered as loops arising 

 and ending in the auriculo-ventricular ring. The course of the 

 fibers in the ventricles has been difficult to make out, and several 

 more or less different accounts have been published. The fibers 

 on the surface of the heart arise from the tendinous rings and mem- 

 branes at the base and take a spiral course to the apex, where they 

 form a vortex and pass into the interior of the left ventricle to enter 

 the septum and make connections with the papillary muscles. 

 In this way they return upon themselves toward the base of the 

 heart and form spiral loops whose contractions serve to approxi- 



