STRUCTURE OF UNSTEIATBD MUSCLE 275 



that their contractions and relaxations are controlled by 

 nerve centers which are not under direct will controlled to 

 their being called involuntary muscles. 



Structure. — The membranes which unstriated mus- 

 cles form are not in bundles like the striated muscles, 

 but are composed of masses of single, spindle-shaped cells 

 stuck together by a cementlike substance and held in 

 place by an interlacing net- 

 work of connective tissue. 

 These cells are only about 



glo inch in length, and ^''- "^ -a single unstriated m^^^^^ 



while they show a longi- 

 tudinal striation there are no cross striations. The cells 

 are covered with a cell wall of transparent mem- 

 ^v, 1 brane and inclose a single nucleus. They are, 

 l^jel therefore, to be considered as single and not com- 

 I \: posite cells. The protoplasm which they contain 

 ~ * ■■ has the power of contraction in the direction of 

 their long axis, and since they are usually arranged 

 about a cavity their contraction and relaxation re- 

 sults in the reduction or enlargement of the cavity. 



Heart Muscle. 



The muscular walls of the heart are composed 

 of a tissue which is intermediate in structure 

 between that of the striated and imstriated 

 Fig. 120 types. It consists of single, nucleated, branched 



— Mus- *"^ 1 



cieceiis cells United to make a meshwork. These cells 



heart!"* show cross striations, but these striations are less 



distinct than those of the skeletal muscles. The 



rate of contraction of the heart-muscle cells is less than 



