CARDIAC MUSCLE 



93 



A smooth muscle fiber with its nucleus and principal cytoplasm body 

 lying outside of the myo-fibril group is described in the wall of the heart 

 vessel of Cerebratulus. Other peculiar contractile cells are found in the 

 pulsating vessels of the worms. Even in these early stages of phylo- 

 genetic development we find a cardiac muscle cell that is somewhat 

 different from the other muscle cells of the body. 



In the majority of the mollusks the heart muscle is composed of 

 smooth cells. In Unio they are simple spindle-shaped forms that are 



FIG. 91. A, heart muscle tissue from the Gasteropod mollusk, Sycotypus. B, transections of 

 other fibers, one through the nucleus and one a short distance from it ; x, body of unknown 

 function, x 700. 



arranged in the peculiar mesh work which is met with in most heart 

 muscles (Fig. 90). Besides their peculiar arrangement, they differ from 

 the ordinary Unio muscle cells in having less fibrillar contractile material 

 and a far larger, central mass of sarcoplasm. In some mollusks, espe- 

 cially those of highest specialization, a well-differentiated cardiac muscle 

 is to be seen. It reaches its highest development in the cephalopods, but 

 the relations are more easily demonstrated in a gasteropod, so we shall 

 'study the heart tissue of Sycotypus canalieulus (Fig. 91). 



But little explanation is necessary. This tissue consists of cells that 

 are spindle-shaped and contain the nucleus in the center of the fiber. 

 Like the heart fibers of Unio, they also have developed the myo- fibrils in 

 the peripheral layer of the sarcoplasm. The prominent difference is, 

 that the myo-fibrils are striated by alternate areas of isotropic and aniso- 

 tropic substance. They also have more of the fibrils developed, giving 

 the cells a heavier and more substantial appearance. 



Striation is evidently a feature that belongs to no particular set of 

 muscle cells but may appear in any of them. This view could not be 

 held by studying the mammalian body alone. The striation is some- 

 times hard to find in Sycotypus. One may examine many differently 

 prepared sections and see no signs of it until, at last, the right part of the 

 right one will show it clearly and indubitably. The striations are more 

 easily .demonstrated in the squid's heart. 



