292 FRANKLIN P. MALL 



it is invaded by nerve cells. From now on there is a change in the 

 structure which is well shown in two foetuses 50 mm. long (Nos. 

 84 and 96) . In both the cells are more ' epithelial' in appearance 

 and in the former this differentiated band can be followed into 

 both of its divisions, one of which reaches to the moderator band. 

 The limb of the bundle which passes to the left ventricle is most 

 sharply defined. In a foetus 80 mm. long (No. 172) this bundle 

 is easily made out and appears much as it does in other foetuses, 

 or as it is in the adult. 



It is thus seen that the atrio-ventricular bundle is present in an 

 embryo 8 mm. long and that as soon as the muscle of the rest of 

 the atrial canal is broken down it becomes outlined by encircling 

 spaces which are now formed. Later it is composed of muscle 

 which resembles much the rest of the heart musculature. When 

 the nerves invade the septum of the atria the atrio-ventricular 

 muscle shows marked histological changes which remind one much 

 of the Purkinje fibers. These I have followed into both ventricles 

 to the moderator band on the right side in the human heart, and 

 through it in the pig. In the foetal sheep up to 15 cm. long much 

 the same conditions prevail and by using a great variety of stains 

 I was able to follow the atrio-ventricular bundle beyond the mod- 

 erator band ; no Purkinje fibers were found. As we now know the 

 continuity of the bundle with the Purkinje system it is not remark- 

 able that in early stages the structure of the main bundle simulates 

 that of this system. In the adult heart Purkinje fibers are found 

 throughout the ventricular portion of the atrio-ventricular bundle 

 and this is all in harmony with what I have found. I can not leave 

 the subject without expressing the suspicion that the differentia- 

 tion of the Purkinje system is in some way due to the influence of 

 nerves when they appear in the wall of the sinus. 



D. MUSCULATURE OF THE LEFT VENTRICLE 



It is practically impossible to unravel the musculature of ven- 

 tricles from serial sections alone. However, it is possible to gain 

 quite a clear picture of the muscle bundles by direct observation 

 of the whole heart with the aid of the dissecting microscope. 

 About two dozen hearts were removed from human embryos 



