VOLUNTARY AND INVOLUNTARY MOTION. 101 



into spindle-shaped cells at death. If this view could 

 also be extended to the smooth muscle masses of other 

 parts, a real connection would exist throughout the 

 muscle-membranes, and the phenomena of the propaga- 

 tion of irritation would admit of a physiological explana- 

 tion. 



6. As a rule, such parts as are provided only with 

 smooth muscle-fibres are not voluntarily movable, while 

 striated muscle-fibres are subject to the will. The latter 

 have, therefore, been also distinguished as voluntary, 

 the former as involuntary muscles. The heart, however, 

 exhibits an exception, for, though it is provided with 

 striated muscle-fibres, the will has no direct influence 

 upon it, its motions being exerted and regulated inde- 

 pendently of the will. 1 Moreover, the muscle-fibres of 

 the heart are peculiar in that they are destitute of sar- 

 colemma, the naked muscle-fibres directly touching each 

 other. This is so far interesting that direct irritations, 

 if applied to some point of the heart, are transferred 

 to all the other muscle-fibres. In addition to this, 

 the muscle-fibres of the heart are branched, but such 

 branched fibres occur also in other places, for example, 

 in the tongue of the frog, where they are branched like 

 a tree. Smooth muscle-fibres being, therefore, not sub- 

 ject to the will, are caused to contract, either by local 

 irritation, such as the pressure of the matter contained 

 within the tubes, or by the nervous system. The con- 

 tractions of striated muscle-fibres are effected, in the 

 natural course of organic life, only by the influence of 



1 Striated muscles also occur in the intestine of the tench 

 (Tinea vulgaris), which in this differs from all other vertebrate ani- 

 mals. It is doubtful whether this tissue ?s capable of voluntary 

 motion, but it is very improbable. 



