MUSCULAR MOTION. 



525 



the contraction has the appearance of being 

 permanent. In the latter case, certain parts of 

 the fibre (as its broken extremities) are fixed 

 more or less firmly, so as to offer a resist- 

 ance to the contraction that takes place, this 

 resistance enabling the contractile force ad- 

 vancing to new parts to obliterate the traces 

 of contraction in the parts in which it is sub- 

 siding, by stretching them. The ends usually 

 become fixed in consequence of their being 

 the first to thicken from contraction and from 

 their thus receiving the pressure of the la- 

 mina of mica or glass with which it is requi- 

 site to cover the object, and they are the first to 

 contract, because irritated both by being broken 

 and also by the water, which is absorbed soonest 

 where the sheath is deficient. This fixing of 

 the ends brings the fibres in question nearly 

 into the condition under which they exist 

 in the living body, where it has already been 

 explained that there is always a resistance to be 

 overcome in active contraction. This particular 

 variety of the phenomenon, therefore, deserves 

 special study. Those animals whose muscles 

 are most tenacious of their contractility are the 

 best suited for examination, and among these 

 the young crab or lobster may be most easily 

 obtained. In an elementary fibre from the 

 claw, laid out on glass, and then covered with 

 a wet lamina of mica, the following phenomena 

 are always to be observed. The ends become 

 first contracted and fixed. Then contractions 

 commence at isolated spots along the margin of 

 the fibre, which they cause to bulge. At first 

 they only engage a very limited amount of the 

 mass, spreading into its interior equally in all 

 directions, and being marked by a close approx- 

 imation of the transverse stripes. These contrac- 

 tions pull upon the remainder of the fibre only 

 in the direction of its length, so that along its edge 

 the transverse stripes in the intervals are very 

 much widened and distorted. These contrac- 

 tions are never stationary, but oscillate from 

 end to end, relinquishing on the one hand what 

 they gain on the other. When they are nume- 

 rous along the same margin they interfere most 

 irregularly with one another, dragging one ano- 

 ther as though striving for the mastery, the 

 larger ones continually overcoming the smaller, 

 then subsiding as though spent, stretched again 

 by new spots of contraction, and again, after a 

 short period of repose, engaged in their turn 

 by some advancing wave : this is the first stage 

 of the phenomenon. (Fig. 302.) The con- 

 tractions increase in number and extent, and 

 gradually engage the whole substance of the 

 fibre. There is still the same struggle, the same 

 alternate action and repose in individual parts, 

 but as the contractions by degrees predominate, 

 the ends of the fibre are drawn more and more 

 near, (intermediate portions by their contrac- 

 tion receiving some of the pressure,) until at 

 last the whole fragment is reduced to a third of 

 its original length, and stiffened with the rigor 

 mortis. 



The muscular tissue in these animals is very 

 tough, but where it is more fragile, as in the 

 Frog, it may give way in the intervals between 

 spots of contraction, and become ruptured and 



Fig. 302. 



Border of an elementary fibre of a ymtnq Crab, shew- 

 ing a spot of contraction (b) and the sarcolemma 

 elevated in the form of bullce by the expressed 

 water (a). Magnified 300 diam. 



disorganized in various degrees.* In fishes I 

 have seen a succession of phenomena similar to 

 what has been described in the Crab ; waves of 

 contraction advancing and receding, but gradu- 

 ally augmenting in bulk, till the whole fibre 

 was finally contracted. (Fig. 303.) 



Fig. 303. 



^gp^rjsr ':- - 



:--'- L_^ ; 



Stages of contraction seen on one occasion in an elemen- 

 tary fibre of the Skate. The uppermost state is that 

 previous to the commencement of contraction. 



a, a, a, &\\cces?,\\e ' w&ves' of contraction seen 

 moving along one margin of the fibre, marked by a 

 bulging of the margin, by an approximation of the 

 tranverse stripes, and by a consequent darkening 

 of the spots. 



b, b, b, similar * waves' still moving along the 

 fibre, but engaging its whole thickness. 



In all these examples, as long as the ends of 

 the fragment are fixed, and will not yield to 

 the convellent force, that force is seen to be 

 exerted in a momentary manner in successive 

 portions of the mass. In proportion as they 

 yield to it, the resistance which enabled the 

 contraction of new parts to stretch those from 

 which it was receding is removed, and the ap- 

 pearances of contraction remain. A distinction 

 is required between the contractile force and 

 the contraction resulting from its exercise. The 

 latter will be permanent, if no force from with- 

 out be exerted to obliterate it by stretching, for 

 a contracted muscle has no power of extending 

 itself; there is no repellent force between its 

 molecules. From these phenomena, therefore, 

 it is possible to eliminate the appearances re- 

 sulting from a subsided force, and to judge of 



* Phil. Trans. 1840, p. 490, pi. xix. fig. 75. 



