HISTOLOGY. 



latter line was not observed by Przewoski. In some prepara- 

 tions this structure cannot be made out. Instead of it, there is 

 a cement line having the characteristic step-like course. This 

 in osmic acid preparations shows the appearance always 

 described as caused by protoplasmic bridges (Fig. 55). 



FIG. 54. 



FIG. 55. 



Longitudinal section of adult hu- 

 man heart muscle, showing the junc- 

 tion of two cells. (MacCallum.) 



Longitudinal section of heart muscle from 

 an adult dog, showing protoplasmic bridges 

 between two cells. (MacCallum.) 



In the lower vertebrates the structure of heart muscle differs 

 in many essentials from that of man and the higher mammals. 

 In fishes the cells are small and spindle-shaped, and possess 

 fibril bundles only around the periphery. In amphibians and 

 reptiles the cells are still spindle-shaped and sometimes 

 branched, but the fibril bundles are more conspicuous 

 (Fig. 49). In birds the heart muscle cell is large and con- 

 tains many fibril bundles. It is differentiated very much more 

 highly than the heart cell of the lower classes of vertebrates. 

 It is a fact worthy of notice that the heart muscle cells of cold- 

 blooded animals are of much more primitive type than those 

 of warm-blooded animals. 



