MUSCULAR TISSUE. 145 



(Bardeen, Godlewski), and mesenchymal tissue, blood-vessels, and 

 nerve-fibers make their appearance between the developing muscle- 

 fibers. 



CARDIAC MUSCLE. 



Cardiac muscle or heart muscle is striated muscle, but differs 

 physiologically and structurally from voluntary striated muscle. It 

 resembles involuntary muscle in that it is not subject to the will. 

 Heart muscle after fixation with many reagents used in the labora- 

 tories, and when treated with macerating fluids, or subjected to the 

 action of silver nitrate, appears to consist of irregularly shaped ob- 

 long cells, cemented end-to-end to form heart muscle-fibers; such 

 fibers appear to anastomose by means of side processes possessed by 

 the cells. A number of recent investigators, notably v. Ebner 

 and M. Heidenhain, have, however, shown that what has been re- 

 garded as cement lines uniting cells are to be otherwise interpreted, 

 since they are known to bound nonnucleated areas of heart muscle, 

 and since the contractile fibrils possessed by heart muscle pass 

 through such lines without interruption. It would appear, there- 

 fore, that heart muscle must be regarded as a syncytium in which 

 no distinct and separate cells occur, but rather of a complex plexus 

 of branching and anastomosing fibers which differ in size and shape. 

 Heart muscle-fibers consist, as was shown for voluntary striated 

 muscle-fibers, of contractile, primitive fibrils, which are grouped into 

 fibril bundles or muscle columns, between which there is found un- 

 differentiated protoplasm, the sarcoplasm. They are surrounded by 

 a sarcolemma, which differs, however, from the sarcolemma of volun- 

 tary muscle-fibers in not being so well developed. The primitive 

 fibrils present the same structure as described for similar fibrils of 

 voluntary muscle, each sarcomere consisting of Krause's membrane, 

 z; two intermediary discs, j; the transverse disc, Q, bisected by 

 Hensen's median disc, H, which in turn contains the median mem- 

 brane of Heidenhain, M. (See Fig. 96.) Krause's membranes (z) 

 and the median membranes (M) extend across the fibril bundles; 

 the former are attached to the sarcolemma (M. Heidenhain). The 

 primitive fibrils are grouped into fibril bundles or muscle columns, 

 which in cross-sections are often band-shaped and are placed radially 

 with reference to the center of the heart muscle-fibers. The 

 sarcoplasm is present in relatively larger quantity than in voluntary 

 striated muscle, especially between the fibril bundles, giving the 

 fibers a distinct longitudinal striation. The primitive fibrils pass 

 uninterruptedly through the anastomoses between the fibers. The 

 nuclei, which are round or oval and possess a distinct chromatin 

 network, are situated near the center of the fibers, occurring at ir- 

 regular intervals, and are surrounded by an axial core of undifferen- 

 tiated protoplasm, in wjjich are found granules which stain in basic 

 stains, also fat droplets, and, especially in older individuals, pigment 

 granules. The structures which have been regarded as intercellular 



