CARDIAC MUSCLE 



95 



But 



is a series of bundles of myo- fibrils in which some sarcoplasm and nuclei 

 have accidentally become entangled rather than a cell with all its normal 

 organs, that contains in addition some myo-fibrils. 



Our illustration (Fig. 92) shows a large developed cell that con- 

 tains about fifteen small bundles of the fibrils. These bundles are 

 subdivided into smaller bundles and the subdivisions contain on a very 

 rough estimate about thirty fibrils on an average. The total cross-section 

 area of the bundles would not be a third of that of the whole cell. 



The sarcoplasm, like that of most muscle cells, is granular, 

 scattered at frequent intervals through the 

 cell are peculiar chromatic bodies, spike- 

 shaped, with the blunt end about as large 

 as a cardiac nucleolus. Each one, where it 

 lies in the sarcoplasm, is surrounded by a 

 clear zone that is visible in the figure. Fig- 

 ure 92 shows four of these bodies, two seen 

 from above and two seen from the side. One 

 of these latter lies very close to a nucleus. 

 The similar bodies in the Sycotypus heart 

 muscle were round instead of spike-shaped. 



The fiber is everywhere covered by a 

 clearly marked sarcolemma, a connective-tis- 

 sue sheath that is made of cells, and con- 

 sequently has its own nuclei. 



A lateral view of one of these fiber por- 

 tions will show several important features. 

 The fibril bundles are practically continuous, running from one mesh 

 to another of the reticulum, sometimes dividing and merging their 

 fibrils with other bundles. 



The fibrils are beautifully striated, and although it is sometimes hard 

 to see the striations, they can be seen in many stages of contraction. 

 The pattern of striation is the same as that of the other lobster muscle, 

 except that the segments are much shorter, and the striations conse- 

 quently much finer and more closely set. The peculiar cells found 

 among the muscle fibers are possibly excretory in function. 



The vertebrate heart, as exemplified by the heart of man, probably 

 shows the most highly specialized cardiac muscle tissue, unless that of the 

 insects can be so considered on account of its very perfect muscle fibers. 



The human cardiac muscle is called a syncytium, although it originates 

 as a mass of mesenchymal cells with their bodies irregularly in contact 

 so as to form a thick-meshed reticulum. In common with the lobster's 

 heart, its striated fibrils run continuously through the reticulum, regard- 

 less of cell boundaries. 



FIG. 93. Several muscle cells 

 from the human heart showing 

 the " intercalated disks " where 

 the bodies touch each other. 

 Nuclei not shown. (After M. 

 HEIDENHAIX. From STOHR'S 

 "Text-book of Histology" by 

 LEWIS.) 



