CHAPTER XIV 



CONTRACTILE TISSUES 



IN the animal organism, the tissues which have the power of effecting movement 

 by changing their form, " contraction," as it is usually called, are known as 

 muscular. It should be made clear at the outset that the word " contraction " is, 

 strictly speaking, incorrect, since there is no change in volume when a muscle 

 becomes active, merely change of shape, by which its two ends are brought closer 

 together. So that, if one end is fixed, the other end moves nearer to it and, if the 

 latter is attached to a movable object, this object moves with it. If the muscle 



is prevented from shortening, owing 

 to attachment to an immovable object, 

 a state of tension is developed in it. 



The mechanism of movement in 

 the plant is of a different kind, and 

 will be described in a special section 

 later. 



There are two kinds of muscular 

 tissue, which, in the extremes of the 

 scale, have very distinct properties, 

 namely, the cross-striated, skeletal, 

 or voluntary muscle on the one hand, 

 and the smooth, non-striated, or in- 

 voluntary muscle on the other hand. 

 There are, however, many degrees of 

 transition between them. The heart 

 muscle of the vertebrate is cross- 

 striated, but exhibits many of the pro- 

 perties of the other class ; the claw 

 muscle of the crayfish is another case. 

 Perhaps the most characteristic 

 difference, physiologically, between 

 the two classes is that the typical 

 skeletal, cross-striated muscle, in its 

 highest form of development, is 

 entirely dependent on impulses from 

 the central nervous system to set it 

 into activity ; the other class pos- 

 sesses an automatic activity, mani- 

 fested in tone, or in rhythmical 

 contraction and relaxation, even when separated from the central nervous 

 system. It is not to be supposed that the involuntary muscle is not subject 

 to control from the central nervous system ; we have seen the contrary to 

 be the case with the intestine, the heart, the blood vessels in the vertebrate and 

 the claw in the crayfish. This last case is, indeed, a difficult one to classify on 

 any system, since, although possessing automatic tone, it is under voluntary con- 

 trol on the part of the anim'al, like the skeletal muscles of the vertebrate. The 

 organs consisting of smooth muscle in the latter organisms, and also the heart, are 

 not under voluntary control, although acted on reflexly. 



Some other differences, rather of degree than of kind, may be mentioned. The 

 rate of contraction of the smooth muscle is usually slow, compared with that 



43 



FKJ. 132. REACTION OF THE MTSCLE OK THE 

 EARTHWORM TO A STRETCHING STIMULUS. 



a to 6 shows the extent to which the lever was pulled down 

 in order to stretch the muscle. 



Time in two-second intervals. 



(After Straub.) 



