The Muscular System 115 



TYPES OF MUSCULAR TISSUE 



In the adult skeleton, the bone tissue is quite inflexible. Muscle 

 tissue, in contrast, is capable of contracting and can be stretched ; thus 

 it has the property of extensibility. After the individual muscle cell 

 has contracted, it can regain its original form without harm to the 

 cell. All cells illustrate this same property to some extent, but never 

 to the degree that is found in muscle cells. Thus elasticity is another 

 important property of muscle tissue. 



Histologically three distinct types of muscle tissue are recognized : 

 smooth muscle, skeletal muscle, and cardiac muscle. 



Smooth Muscle (Fig. 34, B).— The individual muscle fiber or cell 

 is a spindle-shaped, slender body about 0.2 mm. in length. They vary 

 in size from the extremely small fibers found in the walls of blood 

 vessels (0.015 mm. in length) to those found in the uterus. During 

 pregnancy, these latter fibers may attain a length of 0.5 mm. The 

 cytoplasm of these cells is quite homogeneous without the dark and 

 light bands that are seen in other types of muscle tissue. It is for this 

 reason that this type of muscle is known as smooth muscle. The some- 

 what oval nucleus is located in the central widest portion of the cell 

 body. Contractile threads or fibers, myofibrils, run lengthwise of the 

 cell. The cytoplasm between the fibrils is termed sarcoplasm. 



NUCLEUS 



Fig. 34. — Types of muscle tissue. A, Cardiac; B, smooth; C, enlarged 

 section of skeletal; D, skeletal. 



Smooth muscle tissue is found in nearly all the internal organs. 

 The walls of the intestine have layers composed entirely of this tissue. 

 The walls of the bladder, the uterus, the respiratory system, and the 

 blood vessels all have an abundance of smooth muscle tissue. Scattered 

 throughout the skin are many other fibers. The hairs can be raised 

 on the skin by the small cylindrical bundles of smooth muscle located 

 at their bases. 



