B 



OTHER FORMS OF CONTRACTILE TISSUE 275 



PROPAGATION OF THE EXCITATORY STATE, OR WAVE OF 

 CONTRACTION. On stimulating any part of a voluntary muscle 

 fibre, a wave of contraction is started which travels to each end of the 

 fibre, but no further. There is no propagation from muscle fibre to 

 muscle fibre, the synchronous contraction of the whole muscle being 

 brought about by simultaneous excitation of all its fibres. It is doubt- 

 ful whether this isolation of the excitatory state is found in smooth 

 muscle. As a rule a stimulus applied to any part of a sheet of smooth 

 fibres may travel all over the sheet just as if it were a single fibre. It 

 seems probable indeed that there is protoplasmic continuity by means 

 of fine bridge-like processes between adjacent muscle-cells. 

 And even in the absence of such bridges the propagation of 

 the contraction could be easily accounted for. Although in 

 the case of voluntary muscle the rule is isolated contraction, 

 yet a very small change in the muscle, such as that produced 

 by partial drying or by pressure, is sufficient to cause the 

 contraction to spread from one fibre to another. Indeed by FlG - 9 ~- 

 clamping two curarised sartorius muscles together, as in the 

 diagram (Fig. 97), it is found that stimulation of the muscle A causes 

 contraction of the muscle B. The current of action of A in this case 

 has served to excite a contraction in B. 



It must be remembered that in all unstriated muscle the fibres are sur- 

 rounded by a network of non-medullated nerve fibres. Some physiologists 

 are inclined to ascribe to these fibres an important part in the propagation 

 of the contraction wave. In the case of the heart muscle, however, it can 

 be shown almost conclusively that the propagation takes place independently 

 of nerve fibres, and probably the same is true for many kinds of involuntary 

 muscle. 



INFLUENCE OF TEMPERATURE. Smooth muscle is extremely 

 susceptible to changes of temperature ; as a rule warming causes 

 relaxation, while application of cold causes a tonic contraction. The 

 condition of the muscle at any given time does not depend only on 

 its actual temperatuie, but also on the rapidity with which this 

 temperature has been reached. Thus a rapid cooling of the retractor 

 penis muscle of a dog from 35 to 25 may cause a contraction as 

 extensive as would be produced by a slow cooling to 5 C. On warm- 

 ing a muscle from 30 to 50 C. it lengthens gradually up to about 

 40, and it may then undergo a marked heat contraction (varying 

 in degree in different muscles) at about 50 C., which may pass off at a 

 somewhat higher temperature. It is killed somewhere between 

 40 and 50 C. It seems very doubtful whether any true rigor mortis 

 occurs in smooth muscle. The hard contracted appearance of the 

 smooth muscle in a recently dead animal is chiefly conditioned by the 

 fall of temperature. On excising the muscle and warming it up to 



