86 



THE MUSCLE TISSUES 



dissociating fluid, cardiac muscle can be shaken into separate units 

 resembling single cells. They are roughly rectangular in shape, 

 with parallel sides and very uneven ends. One or both ends may 

 have short stubby branches that connected it with other adjacent 

 fibers. Each of these units is cross-striated and shows longitudinal 

 fibrillae. It is not certain that the apparent ends are actually 

 boundaries of cells; they may be artefacts brought about by the 

 treatment. 



The isolated units of cardiac muscle correspond to cells having a 

 nucleus and a surrounding portion of cytoplasm. Howe\er, the 

 most reasonable conclusion drawn from the lack of definite cell 

 boundaries, and the fact that the myofibrils extend continuously 

 through several such units, is that the cardiac muscle is a syncytium. 

 The myofibrils appear to be similar to those of skeletal muscle, 



Fig. 47. — Diagram of cardiac muscle cells, showdng intercalated discs. 



where they are more easily studied. At times even in fresh heart 

 tissue, and particularly after certain techniques, peculiar bands, the 

 intercalated discs, may be obser^'ed extending partially or inter- 

 ruptedly across a fiber. (Fig. 47.) These discs were formerly 

 thought to represent end-boundaries of the cells, but this seems 

 unlikely since some of them pass across a fiber at the level of the 

 nucleus and others delimit portions of a fiber without a nucleus. 

 They usually present a staircase appearance and the separate 

 portions do not o^'erlap. Some investigators interpret them as 

 places where the fibers were in a state of contraction. They are 

 more commonly found in older cardiac tissue and from this the 

 inference has been drawn that possibly they represent lines where 

 normal functioning is breaking down. The exact nature of these 

 structures remains to be demonstrated. 



The blood supply for cardiac muscle is supplied directly from 

 the clefts between the fibers in the case of the lower vertebrates, 



