DEVELOPMENT OF THE HUMAN HEART 



293 



ranging from 10 to 40 mm. in length and these were first carefully 

 studied with various enlarging lenses including the binocular. It 

 was found that the hearts from embryos over 15 mm. long could 

 be dissected under the binocular to great advantage, especially 

 after they had been stained in alum cochineal. So all of the 

 hearts were stained and preserved in alcohol. No dissection was 

 made hastily and during the preparation numerous sketches made. 

 In the course of time after many attempts had been made I finally 

 satisfied myself regarding the various stages of the development 

 of the chief muscle bundles of the left ventricle. 



The following specimens were dissected; the smallest embryo 

 was an unusually good one from a tubal pregnancy. The heart 



Number and length of etnbryos whose hearts were removed entire, studied and dissected 



was removed and examined many times in various fluids by trans- 

 mitted and reflected light. The best drawings were obtained 

 from the stained heart while it was in 60 per cent alcohol. These 

 are shown as diagrams in figs. 31, 32 and 33. It is noticed at once 

 that most of the striations on the surface of the heart are circular, 

 which is probably the arrangement of all the fibers in younger 

 specimens. However, at the apex certain changes have taken 

 place which fully account for the arrangement of the muscle 

 bundles in the adult heart. In front of the heart, at the bulbo- 

 ventricular furrow, the fibers leave the bulb and penetrate the 

 medial side of the left ventricle, as indicated in fig. 31. Behind 

 the fibers leave the surface of the left ventricle and enter the 



