58 GENERA t CONCEPTS 



Nuclei 



Cross Sfriotions 



A, SKELETAL MUSCLE FIBERS 



Nuclei 



B, SMOOTH MUSCLE FIBERS 



Nuclei 



C, CARDIAC MUSCLE FIBERS 

 Figure 3.13. Types of muscle tissue. (Villee: Biology.) 



cells has many nuclei. The nuclei of skeletal muscle cells have an un- 

 usual position, at the periphery of the cell, just below the plasma mem- 

 brane. Skeletal muscle cells are extremely long, an inch or more in 

 length; indeed, some investigators believe that the muscle cells extend 

 from one end of the muscle to the other, so that their length is equal 

 to that of the muscle. Muscle fibers range in thickness from 10 to 100 

 microns; continued, strenuous muscle activity increases the thickness of 

 the fiber. The myofibrils of skeletal and cardiac muscle have alternate 

 dark and light cross bands or striations. These appear to have some 

 fundamental role in contraction, during which the dark stripes decrease 

 in width and the light stripes increase in width. The contraction of 

 skeletal muscles is generally voluntary, under the control of the will, 

 that of cardiac and smooth muscles is involuntary. Cardiac muscle cells 

 are striated but have centrally located nuclei. Smooth muscle cells are 

 not striated, have pointed ends, and have centrally located nuclei. 

 Smooth muscle contracts slowly but can remain contracted for long 

 periods of time. In some of the invertebrates the voluntary muscles of 

 the body, such as the ones which close the shell of an oyster, are smooth 

 muscles. Striated muscles can contract very rapidly but cannot remain 

 contracted; a striated muscle fiber must relax and rest before it is able to 

 contract again. The muscles of insects, spiders, crabs and other arthro- 

 pods have cross striations and contract very rapidly. The distinguishing 

 features of the three types of muscle are summarized in Table 1. 



Vascular Tissues. The blood, composed of a liquid part— plasma 

 —and of several types of formed elements— red cells, white cells and 

 platelets— may be classified as a separate type of tissue or as one kind of 



