274 MUSCLES 



protoplasm. The oval bodies are nuclei. The faint, 

 lengthwise striations indicate a peculiar, structural divi- 

 sion of the sarcoplasm. Examination shows that it is 

 divided into long hardened portions or fibrils, which 

 extend lengthwise of the ceU, and the spaces between these 

 fibrils are filled with a liquid portion more like ordinary 

 protoplasm. It is the faint outlines of these fibrils that 

 give the lengthwise striations. These columns are also 

 called sarcostyles to distinguish them from the liquid 

 sarcoplasm. Each sarcostyle is divided transversely into 

 disks of alternate dark and bright appearance, and these 

 combined disks are what give the cross striated appear- 

 ance. Both the sarcostyles and the sarcoplasm are merely 

 modified forms of protoplasm. The part played in muscle 

 action by these various elements of the fiber cell will be 

 discussed later under muscle action. (See p. 278.) At 

 present, all we need note is that the substance of a 

 muscle cell has a highly specialized power of contraction, 

 and that the combined contractions of many cells united 

 into bundles and these into other bundles, is responsible 

 for the ultimate action of the muscle, just as the combined 

 pull of many men in a tug of war results in one motion of 

 the rope. Most striated muscles are under the control of 

 the will and were formerly included under the heading 

 of voluntary muscle. 



Unsteiatbd Muscles. 



Muscles of this class are found in the walls of the diges- 

 tive tract and organs, the blood vessels, the genital and 

 urinary organs, etc. Such muscles show no tendons, but 

 form expanded membranes that surround cavities. There- 

 fore they have no definite origin or insertion. The fact 



