HISTOGENESIS AND STRUCTURE 



103 



seem to represent a younger or less highly differentiated stage of muscle 

 fiber. In cross section they are of greater diameter, with fewer peripheral 

 myofibrils and a far greater amount of central undifferentiated sarco- 

 plasm, rich in glycogen. According to Lange (Arch. mikr. Anat., Bd. 

 84, 1914) the Purkinje fibers cannot, however, be regarded as remains of 

 embryonic muscle cells, since they are clearly distinguishable already in 

 very young mammalian embryos ; they constitute the 

 non-nervous apparatus for conducting stimuli to heart 

 beat. 



The myogenic theory accordingly seemed well es- 

 tablished. It was apparently strongly supported by the 



experiments of Erlanger (1906), who clamped the 



bundle in the dog's heart and produced a condition of 



'heart block' a disturbance of the coordinated rhyth- 



micity of the atria and ventricles without, however, 



interfering with the conduction of impulse, since there 



resulted no stoppage of contraction in the ventricles. 



But the subsequent discovery of abundant nerve fibers 



and ganglion cells (Tawara, Wilson, McGill) in the 



bundle, intimately related to the cardiac fibers, robbed 



this experiment of its specific applicability and seemed 



for a time to force an interpretation favoring the neu- 



rogenic theory. Carlson, moreover, demonstrated its 



validity for the Limulus heart, which, after the 



removal of its ganglion, could not be made to beat. 



But more recently Burrows (1911) has shown that 



single cells of a 14- to 18-day embryo chick heart, 



grown in artificial culture media, may begin to beat 



automatically and rhythmically an observation which 



would seem to settle the point that heart muscle may 



beat rhythmically in the absence of nerve supply or 



even nerve stimulus. 



Furthermore, Hooker (Jour. Exp. Zool., 11, 2, 1911) showed that in the 

 frog larvae in which the nervous system was entirely removed, the cardiac 

 muscle will differentiate and function normally, independently of nervous 

 control. The myogenic theory is further supported by the fact that it has 

 been possible to revive the excised heart of man to rhythmic activity 20 

 hours after death (Flack, "Further Advance in Physiology," Ed., Hill, 1909), 

 whereas the longest time that a nerve cell is known to survive (in the intes- 

 tine) is 3V 2 hours (Cannon and Eurkeit, Amer. Jour. Phys., vol. 32, p. 347, 

 1913). In the superior cervical ganglia, nerve cells may survive 1 hour, 

 while in the brain the maximum time of survival is said to be 15 minutes. 



FIG. 113. LONGI- 

 TUDINAL SECTION 

 OF A TRABECULA 

 OP LIMULUS 

 (KING CRAB) 

 HEART MUSCLE, 

 SHOWING AN IN- 

 TERC ALATED 

 DISK SEPARAT- 

 ING A CONTRACT- 

 ED FROM AN UN- 

 CONTRACTED 

 PORTION. 



Nitric acid-alco- 

 hol fixation (Zlm- 

 mermann's tech- 

 nic), iron-hematox- 

 ylin stain. X 1300. 



