88 HISTOLOGY. 



3. Voluntary Striated Muscle (Skeletal). 



This tissue is made up of the most highly differentiated of 

 all muscle cells. They are long fibres, possessing a sarco- 

 lemma or cell membrane, many nuclei, and a protoplasm con- 

 taining fibrils with a double striation (Fig. 58). Each cell is 



Part of voluntary muscle fibre of frog. 



what is known as a syncytium i. e., the n-ucleus has divided 

 many times without corresponding division of the cell. They 

 may be as long as 10 cm., and in small muscles may extend 

 their entire length. The diameter of the cells varies from 30 

 to 60 p. In old animals the fibres are larger than in young. 

 When the ending of a fibre is found, it is seen to be conical or 

 round. Often the ends are branched or forked, as in the 

 muscle fibres of the tongue. 



The cross-striation is due to strias which are present in the 

 longitudinally disposed fibrils (Fig. 60). These can be seen in 

 the fresh muscle. The fibril bundles are made up of what are 

 called primitive fibrils. These are differentiated parts of the 

 protoplasm, and have to do with the power of contraction pos- 

 sessed by the cell. A small part of the protoplasm remains 

 unchanged, the so-called sarcoplasm. The arrangement of the 

 fibrils may be variable, as seen in a cross-section. They may 

 form polygonal bundles, which are known in cross-section 

 as Cohnheim's fields (Fig. 59). These correspond to what 

 v. Kolliker has termed Muskelsaulchen in heart muscle. They 

 are separated by more or less sarcoplasm, which appears as ;i 

 bright network, in whose meshes the fibril bundles or Cohn- 

 heim's fields are situated. The distinctness of the striations 



