CARDIAC MUSCLE 



])eripheral cytoplasm. Tliese orraniiles increase in number and appear 

 to become arrangetl in linear fashion and fuse to form coarse fibers, 

 the cardiac myofibrils. The cells grow in length but preserve their 

 fiber-like form and lateral attachments with adjoining myoblasts. 

 (Fig. 4(>.) The myofibrillar increase in number presumably b\- longi- 

 tudinal di\ision and become more numerous in the peripheral 

 region, leaving a central portion with a core of undifferentiated 

 cytoplasm and the nucleus. With dcNelopment the fibrils show 

 striations resulting from alternate differences in composition; these 

 appear as dark and light bands which occur at the same level in all 

 the fibrils of a given fiber, so that the whole fiber has a striated ap- 

 pearance. In the lower vertebrates the myofibrils are less numerous 

 and form a peripheral layer, 

 but in higher forms they are 

 scattered throughout, except 

 in the immediate region of the 

 nucleus. 



The fibers of higher verte- 

 brates are limited by a thin 

 membrane called the sarco- 

 lemma, which is usually con- 

 sidered to be a condensation of 

 sarcoplasm. Among fishes and 

 amphibians, cardiac muscle 

 appears to lack the interstitial 

 tissue found in higher forms, 

 and the bundles of muscle 

 fibers are separated from the 



blood only by the covering endothelium, a condition resembling 

 the embryonic state of higher forms. 



A longitudinal section of cardiac muscle does not have the appear- 

 ance of separate cells as does smooth muscle. The general picture 

 is that of a network of long fibers in which the nuclei are at regular 

 intervals in the center of the fibers. The clefts between fibers are 

 small in the higher vertebrates but are easily seen in lower forms, 

 such as the fish and frog. A cross-section shows sections of the 

 fibers irregular in outline in places where branching occurs, but in 

 other portions of the fibers the size is more uniform and the outline 

 quite regular. This is in contrast with the marked variability in 

 cross-sections of smooth muscle, where the outlines are regular but 

 the size varies with the region of the cell cut. After treatment in 



Fig. 46. — Photograph of an isolated 

 portion of cardiac muscle of the frog, 

 showing a fiber with a central nucleus, 

 striated myofibrils, aud branches. 



