128 



HISTOLOGY 



CARDIAC MUSCLE. 



A portion of the mesenchymal syncytium from which cardiac muscle 

 develops is shown in Fig. 120. Its nuclei are found in the axial part of the 

 protoplasmic strands, at varying intervals from one another. Peripherally 

 a few myofibrils have developed from the chondrioconta, or protoplasmic 

 granules, and these fibrils extend for considerable distances through the 

 syncytium regardless of cell areas. They multiply rapidly, and form a 

 peripheral layer of fibrils surrounding the central nuclei and axial proto- 

 plasm. Thus as seen in cross section, the strands of cardiac syncytium 



and the myoblasts of skeletal muscle 

 resemble one another. The fibrils 

 exhibit alternating dark and light 

 bands which are arranged as in 

 skeletal muscle, and " ground mem- 

 branes (Z) develop across the fibers, 

 bisecting the light bands (/). The 

 striations, however, are not as regular 

 and as highly developed as in 

 skeletal muscle. At the periphery 

 of the fibers there is a sarcolemma, 

 which is thinner than that of skeletal 

 muscle, and was formerly overlooked. 

 In early stages the muscle fibers in 

 many places rest close against the 

 endothelium of blood vessels; later 

 they are surrounded by more or less 

 connective tissue. 



In the adult the cardiac muscle fibers anastomose freely, thus retaining 

 their original syncytial arrangement (Fig. 121). They do not, however, 

 form an irregular network, but are arranged in layers, in which the fibers 

 tend to be parallel. Thus they are cut longitudinally in Fig. 121 and 

 transversely in Fig. 172 (p. 179). The nuclei retain their central position. 

 They are elliptical bodies with a conical mass of protoplasm at either pole. 

 This protoplasm, as in smooth muscle, occupies the interval left between 

 the fibrils as they diverge to pass by the nucleus. It is granular, and 

 frequently contains brown pigment. 



According to Apathy (Biol. Centralbl., 1888, vol. 7) "the contractile substance 

 is a product of the muscle cell and the muscle cell is represented by the nucleus and 

 surrounding area of protoplasm." "The myofibrils of the contractile substance are 

 the histogenetic homologues of connective tissue fibrils, however much they may 

 differ from them chemically or functionally." Baldwin has recently advanced a 

 similar interpretation. He finds that the sarcoplasm between the fibrils differs from 

 the protoplasm around the nucleus. Moreover he states that the perinuclear proto- 



FIG. 120. CARDIAC MUSCLE FROM A DUCK 

 EMBRYO OF THREE DAYS. (M. Heidenhain, from 

 McMurrich's "Embryology.") 



