536 CIRCULATION OF BLOOD AND LYMPH. 



by way of the septum on the dorsal side of the aorta. These 

 fibers in the developed heart make a strong circular system whose 

 contraction tends to diminish the lumen of the left ventricle. 

 Below the superficial sinospiral system lies the deep sinospiral 

 sheet (S S f ), which arises from the posterior side of the left ostium 

 and passes transversely to enter the interior of the right ventricle 

 and then turn upward toward the base. At the base of the heart 

 some of the fibers of the bulbospiral system pass circularly round 

 the base of the aorta and the left ostium, and in the right ventricle 

 some of the sinospiral system form circular loops round the conus 

 at the base of the pulmonary artery. As will be described below, 



/CLV 



Fig. 223. Posterior view of heart, somewhat to left, after the superficial sinospiral band 

 has been removed to the posterior longitudinal sulcus: BS f , deep bulbospiral band; BS, super- 

 ficial bulbospiral band, A, B, and C are fibers belonging to this system and forming the posterior 

 horn of the vortex ; SS, superficial sinospiral band, D and E are fibers belonging to this system 

 and forming the anterior horn of the vortex; CLV, the circular band (bulbospiral system) round 

 the left venous ostium (Mall). 



there is some physiological evidence to indicate that this latter 

 group of circular fibers around the base of the pulmonary artery 

 and aorta are the last to enter into contraction in the systole of the 

 ventricle, as might be expected from their homology with the 

 musculature of the bulbus arteriosus in the hearts of the lower 

 vertebrates. 



The Auriculoventricular Bundle. A matter of very great 

 physiological interest in connection with the invariable sequence 

 of the heart-beat has been the question of the existence of a direct 

 muscular connection between the auricles and ventricles. In 

 the lower vertebrates there is muscular continuity throughout 

 the heart from the venous end to the arterial end. In hearts of 

 this type (see Fig. 224) we may distinguish four different chambers 

 the sinus venosus, into which the great veins open, the auricle 



