60 



CONSCIOUS NERVOUS OPERATIONS 



HUH 



Fig. 40. Pieces of 

 striated muscle 

 fibers. 



Showing the spin- 

 dle - shaped nuclei 

 and cross-striations. 

 At the left above is 

 the rounded end of a 

 fiber. 



69. Classification. The muscles are divided into two 

 classes, usually called the voluntary and the involuntary 



muscles, the first being under the con- 

 trol of the will, while the second are not. 

 There is also a difference under the 

 microscope between the two classes of 

 muscles. Voluntary muscular tissue is 

 composed of fibers which are marked by 

 alternate dark and light stripes. They 

 are called striated or striped muscular 

 fibers (Fig. 40). The fibers which com- 

 pose the involuntary muscles are, as a 

 rule, destitute of these markings and 

 are called plain muscular fibers (Fig. 

 41). 



Certain exceptions to the above rule 

 should be noted. The muscles of the heart, though not 

 under the control of the will, are made up of striped 

 muscular fibers ; and also the muscular fibers found in 

 the pharynx, part of the esophagus, and in the internal 

 ear, though involuntary, have the structure of voluntary 

 muscle fiber. On the other hand, the ciliary muscles, 

 by which the eye is accom- 

 modated for seeing objects 

 at different distances, are 

 under the control of the 

 will, though composed wholly of plain or unstriped 

 muscle fibers. 



Some striped muscles, like those of respiration and of 

 the eyelids, are partly voluntary and partly involuntary. 



70. Voluntary Muscles. These are also called skeletal 

 muscles, because they constitute the muscular apparatus 

 attached to the bones. Each muscle is usually larger in 



Fig. 41. Plain muscle fiber. 



N nucleus. 



