GENERAL HISTOLOGY. 91 



elements, the muscle-fibres and muscle-fibrils, on the contrary, 



can shorten only by correspondingly increasing in 



diameter (fig. 47); they can therefore accomplish 



motion only in a definite direction, that of the axis 



of the muscle. The muscle-substance consequently 



is more limited in its movement than is protoplasm, 



but on the other hand it has the advantages of 



greater energy and greater rapidity. An observer 



conversant with the different kinds of motion is 



able to decide with considerable accuracy, from the 



intensity and rapidity, whether in a given case a 



movement has been brought about by the agency of 



protoplasm or by the contractile substance in the 



narrower sense (muscle-substance). tracted state. 



Formation of Muscle-substance. These physiological con- 

 siderations show that protoplasm and the contractile substance are 

 morphologically different, and that therefore one must distinguish 

 sharply between formative cells, or muscle-corpuscles, and the 

 product of these cells, the contractile substance, just as in the 

 case of connective tissue, between the connective-tissue corpuscles 

 and the connective-tissue fibrils. This distinction actually occurs, 

 but optically it is not equally demonstrable, for the reason that it 

 is not prominent histologically. In animal histology there are 

 recognized two kinds, it might even be said two stages, in the 

 formation of muscle-substance, the homogeneous, or smooth, and 

 the cross-striated. Since the former looks very similar to non- 

 granular protoplasm, the boundary-line between it and the 

 muscle-corpuscle is more difficult to recognize than in the case of 

 the cross-striated muscle-substance, which in its minute structure 

 is quite different in appearance from protoplasm. In cross-striated 

 muscles the contractile portion consists of two substances regularly 

 alternating with one another in the direction of the contraction of 

 the muscle, of which the one is doubly, the other singly, refractive 

 (figs. 24, 47, 50). 



Smooth and Cross-striated Muscle-fibres. The smooth muscle- 

 substance represents a lower stage of development than the cross- 

 striated, since it chiefly occurs in the less highly organized and 

 more inactive animals. Interesting in this respect is the fact that 

 in the two stages of development of one and the same animal the 

 simple and inert polyp has smooth muscles, while the more highly 

 organized and actively motile medusa has cross-striated muscles 

 (fig. 48). The difference in their action has led in the vertebrates 



