THE TISSUES OF MOTION 



77 



FIG. 78. Transverse section of a striated muscle 

 fiber from the bat's tongue. To show the com- 

 paratively great amount of sarcoplasm. x i 200. 



striation or segmentation consists of a transverse division of the body 

 of the fibril into a larger or smaller series of equal w.orking units called 

 muscle elements, sarcous elements, or sarcomeres, and these have a defi- 

 nite structure which is exactly the same in all the fibrils in a given cell. 

 Each sarcous element itself has a definite and symmetrical segmen- 

 tation which varies in different forms, and changes during the con- 

 traction and expansion of any 

 given example. This shall be 

 described in detail further on. 



The non-striated fibrils are 

 perfectly smooth. In some 

 mollusks a peculiar woven ap- 

 pearance of the fibers is prob- 

 ably due to the irregular 

 arrangement of its weakly stri- 

 ated fibrils. 



Some muscle cells may con- 

 tain but few of the fibrils 

 scattered singly through the 

 sarcoplasm. Again, in others, 

 the fibrils are more numerous 

 and are gathered together into 



a number of column-like (see Fig. 92, lobster's cardiac muscle) or 

 plate-like groups (see Fig. 82). These groups will be spoken of as fibril- 

 bundles. When cut in transverse sections they are known, in mamma- 

 lian muscle, as Conheim's fields. In the bat the fibrils are all 

 separated and are not so grouped (Fig. 78). Sometimes the fibrillae 

 are so numerous and form such a homogeneous mass that they fill the 

 greater part of the cell, only leaving room for the nucleus and a single 

 cone-shaped area of sarcoplasm at each end of it. 



Each muscle fibril is usually surrounded by a differentiated zone of 

 sarcoplasm, which may be called the cement substance. This cement 

 substance is common to all the fibrils in a bundle, in some forms of 

 muscle (see Fig. 82). In others it is not, and in some it is not apparent 

 at all. 



Any conceptions of the cause of the mechanism of motion that we 

 may entertain must center around the fibril. Have we here an inten- 

 sification of the same phenomena that occur in an Amceba when it con- 

 tracts, or was the first fibril a new organ that enabled its possessor to do 

 more than the life forms had been able to do up to that time ? 



Engelmann has given us, so far, the best theoretical explanation of 

 the motion of the fibrillated muscle cell. In this explanation the fibril 

 is supposed not to have any "vital" power of moving, but to be an inert 



