THE MECHANISM OF TRANSPORT 



123 



the sarcolemma often richly impregnated with fine granules. The 

 sarcoplasm and nuclei represent the remains of the primitive cells 

 (myoblasts) from which the heart is developed; the fusion of these 

 forms a syncytium in which the fibrils develop. These are the true 

 contractile elements of the muscle fibres. They are somewhat pris- 

 matic in shape, and lie in bundles at the periphery of the fibre, the 

 centre being occupied by the nucleus and sarcoplasm. The sarcostyles 

 exhibit a longitudinal striation due to the fibrils, and sometimes a 

 transverse striation due to the presence of singly and doubly refractile 

 substances alternately placed within the fibrils (Fig. 32). 



FIG. 32. MUSCULAR NETWORK OF NORMAL HEART OF ADULT MAN. (Przewoski.) 



a, Septum; b, fibrils passing through thickenings in fibre; c, nuclei of cells; d, short 

 segment without nucleus. (From " Quain's Anatomy.") 



There is an intimate fusion between neighbouring fibres ; a number 

 of fibrils from one fibre pass into a neighbouring fibre. 



At intervals transverse lines appear in the fibres. Some think 

 these are caused by a local thickening of the fibre produced by the 

 process of death; others believe they have some special function in 

 regulating the growth of the fibres. They are not to be regarded as 

 a cement substance separating different heart cells. 



The Nervous Elements of the Vertebrate Heart. The vertebrate 

 heart is very rich in nervous elements ganglion cells, nerve fibres, 

 and nerve endings. There is a very rich supply of ganglion cells in 

 the auricle. In the frog's heart they are grouped at the sinu-auricular 



