THE HEART BEAT. 533 



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'), 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. 



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

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

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

 horn of the vortex; i'S, superficial sinospiral band, D and jB^ 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). 



The Nodal Tissue and Conducting System. — A matter of very 

 great physiological interest in connection with the invariable se- 

 quence of the heart-beat has been the question of the part of the 

 heart in which the beat begins and the paths by which this initial 

 impulse is conducted to the other parts of the heart. 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 

 (right and left), the ventricle (single), and the bulbus arteriosus 

 or bulbus cordis. The musculature of each chamber connects 

 with that of the succeeding one, and the contraction wave, which 

 begins in the sinus, spreads in order to the following divisions of 



