PLAIN MUSCULAR TISSUE. 



75 



are all more or less under the control of the will ; we 

 can make them contract or prevent this as we choose ; 

 they are therefore often called the voluntary muscles.* 

 There are in the body other muscles whose contractions 



FIG. 34. The muscular coat of the stomach. 



we cannot control, and which are hence called involuntary 

 muscles ; they are not attached to the skeleton directly, 

 nor concerned in our ordinary movements, but lie in the 



"What by involuntary? Which kind is attached to the skeleton? 

 Where do we find the involuntary muscles? 



* No sharp line can be drawn between voluntary and involuntary muscles ; the 

 muscles of respiration are to a certain extent under the control of the will ; any one 

 can draw a long breath when he chooses. But in ordinary quiet breathing we are 

 quite unconscious of their working, and even when we pay heed to it our control of 

 them is limited ; no one can hold his breath long enough to suffocate himself. In- 

 deed, any one of the striped muscles may be thrown into activity, independently of 

 or even against the will, as we see in the "fidgets" of nervousness, and the irre- 

 pressible trembling of extreme terror. Functionally, when we call any muscle vol- 

 untary, we mean that it may be controlled by the will, but not that it necessarily 

 always is so. Structurally, the heart occupies an intermediate place : its striped 

 fibres resemble much more those of voluntary than of involuntary muscles, but its 

 boat is not at all subject to the will ; though, as the exception proving the rule, it 

 may he noted that there is an apparently well-authenticated case of a person who 

 could by an act of will stop his heart. 



