DEVELOPMENT OF THE HUMAN HEART 



291 



in another specimen of the same size and prepared in the same way 

 (No. 43, 18.5 mm. long). At this stage the nerves have reached 

 the region of the sinus as is seen in specimen (460, 21 mm. long). 

 In this specimen the bundle can be followed by the spaces around 

 it, figs. 27 and 28. Another specimen (368, 20 mm.), which is 

 unusually well preserved to show the fibrillae of the heart muscle, 

 shows that the degree of development of the atrio-ventricular 

 bundle is the same as that of the rest of the musculature of the 

 heart. 



30 



Fig. 29 Coronal section to show the position of the atrio-ventricular muscle. 

 Embryo 16 mm. long (No. 409). X 18. 



Fig. 30 Coronal section, well dorsalwards, to show the extension of the atrio- 

 ventricular muscle to the coronary sinus. Embryo 15.2 mm. long (No. 423). 

 X 15. 



The spaces around the bundle in embryos from 10 to 20 mm. 

 long appear to continue in the adult; possibly they form the bursa 

 of the bundle described by Curran 38 and the spaces which encircle 

 the Purkinje fibers. Brilliant injections of these have been made 

 by Lhamon. 36 



In an embryo 34 mm. long, No. 249, the atrio-ventricular bun- 

 dle is chiefly muscular as it passes to the ventricle. Near the sinus 



36 Curran, A constant bursa in relation with the bundle of His. Anat. Rec, 

 vol. 3, 1909.- 



36 Lhamon, The sheath of the sino-ventricular bundle. .Amer. Jour. Anat., 

 vol. 13, 1912. 



