96 HISTOLOGY 



And yet in the human tissue this continuity of the fibrils is only func- 

 tional, the fibrils being broken at the cell boundaries by the interposition 

 of small (probably non-contractile) intercalated disks (Fig. 93). We 

 must remember that where two cells, provided with fibrils developed to 

 sustain a strain, join each other in the line of that strain, the fibrils of 

 each must join with those of the other, or they will not be able to perform 

 their function. They would then soon atrophy from disuse. Read 

 the discussion of the lobster's ligament tissue for its bearing on the 

 fibrLlar continuity of joining cells (see Fig. 67 and description). 



Each fibril, in the distinct individual cells of the mammalian heart 

 muscle, joins the fibrils of an adjoining cell in order to have a strong and 

 functional point of attachment. It is very doubtful if this muscle cell 

 supports any but its own fibrils trophically, or even furnishes them a nerv- 

 ous stimulus, although this latter case is more probable than the first. 



Each cell is possessed of a single large nucleus, as a rule, although 

 there may be two. In many cases the appearance leads one to think that 

 an amitotic division is taking place, which is not probable. The cells 

 are branched at an acute angle, and by joining their short processes with 

 the other muscle cells they form the muscular reticulum. The cell 

 boundaries are sharply marked by the rows of " intercalated disks," 

 which so resemble one of the striations that unless stained specially they 

 are not easily seen. A connective-tissue sarcolemma invests all parts of 

 the cardiac fiber reticulum. Its narrow, dark nuclei form a sharp con- 

 trast to the full-bodied oval nuclei. 



The striation of this muscle bears the same relation to the body muscle 

 in man that the two bear in the lobster; it is the same in structure but 

 much finer. Because of this it is sometimes a little hard to demonstrate. 



About the only two generalizations that we can extract concerning 

 the cardiac muscles are : first, the cells form an irregular reticulum which 

 can be explained on mechanical grounds ; and secondly, the striation or 

 segmentation of the fibrils is finer than that of the other body muscles. 

 The latter feature probably has some unknown physiological significance. 



Technic. The technic of cardiac muscle hardly differs from that of 

 ordinary striated muscle. The tissue is a little more apt to become brittle 

 and a little more difficult to stain. The intercalated disks are brought 

 out by the use of nitrate of silver. 



LITERATURE 



MAQCALLUM, J. B. "On the Histology and Histogenesis of the Heart-muscle Cell," 



Anal. Am., 1897. 

 HEIDENHAIN, M. " Uber die Structur des menschlischen Herzmuskels," Anat. Am., 



1901. 



