478 



CIRCULATION OF BLOOD AND LYMPH. 



shorter and its diastole 

 longer than the similar 

 conditions in the ventricles. 

 The Musculature of the 

 Auricles and Ventricles. 

 Embryologically the four- 

 chambered heart is devel- 

 oped 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 super- 

 ficial 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 preceding layer. These fibers may be considered 

 as loops arising and ending in the auriculo-ventricular ring. The 



Fig. 202. Musculature of the heart. (Mac- 

 Callum.) Heart as seen from the posterior: LAV, 

 Left auriculo-ventricular opening; RAV, right 

 auriculo-ventricular opening ; PA, opening of pul- 

 monary artery. I. The superficial muscle layer 

 originating in the right and left auriculo-ventric- 

 ular rings and posterior half of tendon of conus. 

 //. The superficial layer originating in the anterior 

 half of the tendon of the conus (fills in the gap of 

 Fig. I). 77/. The scroll fibers in several layers 

 forming the deeper strata of the heart's muscu- 

 lature. 



