TIIK HKAKT 



43 



staining fibres with larger nuclei. The septal divisions are re- 

 cognised by similar differences. In the moderator band of a 

 human heart true Purkinje cells are not usually found, but the 

 terminal ramifications can be easily seen. The greatest help in 

 tracing the bundle is a knowledge of its course as determined 

 by repeated dissections. This having been obtained, the " lie " 

 of a section will be more readily appreciated, and the probable 

 site of the A-V bundle located. 



The Auricles of the Human Heart. In the primitive heart 



Flo. 4. To show the antagonistic action of the musculatures of the right 

 auricle and ventricle (Keith). A, the jwsition of the A-V groove at the end of 

 auricular systole ; B, its {>osition at the end of ventricular systole. 



depicted in Fig. 1 it is seen that the common auricle is a well- 

 marked outgrowth from the dorsal wall of the auricular canal. 

 Its ostium is indicated by a ring of thick circular musculature 

 (Fig. 1, 2.2). In the mammalian heart the development of the 

 basal wall of the auricular canal has led to a separation of the two 

 parts of the true auricle, namely, the appendices of the right and 

 left auricles. Their original continuity, however, is still preserved 

 by a ridge of musculature passing from the right auricle in front 

 of the termination of the superior vena cava to the left auricle. 



