IX 



CARDIAC MUSCLE AND NERVES 



313 



extremity to other similar cells by means of little protoplasmic 

 bridges (Przewoski), and a series of such cells constitutes a cardiac 

 fibre, which never has a sarcolenima, arid is joined by the processes, 

 as above, to other adjacent fibres, making with them a kind of 

 network. At certain points, more particularly beneath the 

 endocardium, are muscle cells that are non-striated, or striated 

 only in the outer layers, with no striae at the nucleus (Purkinje). 

 These represent cells which are less well developed and more 

 embryonic in char- 

 acter (Fig. 138). 



Given this struc- 

 ture of the ele- 

 ments of the myo- 

 cardium, it is easy 

 on the myogenic 

 theory to see how 

 the contraction 

 wave which arises in 

 the more automatic 

 muscle cells of the 

 venae cavae and 

 sinus venosus must 

 be propagated in a 

 peristaltic form 

 from cell to cell, in- 

 dependent of the 

 nervous system. 

 Each cell being in 

 simple protoplasmic 

 continuity with all 

 the rest, the entire 

 myocardium may, 



f -i -I -I FIG. 138. Muscular network of normal heart of adult man. (Prze- 



'OmapnySlOIOglCai woski.) a, Terminal granular layer; 6, filiform protoplasmic 



nnint- nf VIPW VIP processes, stretched between the muscle cells ; c, nuclei of these 



ju UL .w, MO cells ; d, bundle of primitive muscle fibrils. 



regarded as a united 



mass of hollow muscle. We must now briefly enumerate the 



most important experimental facts by which this theory is 



supported. 



As early as 1874 A. Tick showed that any excitation due to 

 stimulation of a circumscribed area of the cardiac muscular mass 

 was propagated in every direction. Engelmann almost simul- 

 taneously confirmed this fact, and further showed that the con- 

 traction can be propagated in a ventricle divided into zigzags by 

 incisions, from one section to another. 



Porter (1899) established a similar fact for the heart of 

 mammals, irrigated with defibrinated blood circulated through the 

 coronary arteries, at 36 C. After cutting across the mass of the 



