180 LOCOMOTION. [CHAP, vn 



neighbourhood. The accidental circumstances under which the 

 fragments are placed, explain these varieties. In the former case, 

 Fig. 49. the ends of the fragments hap- 



pen to be freely moveable, and 

 are drawn towards each other, 

 according to the amount of con- 

 traction occurring in particular 

 spots; and, as the contractile 

 force leaves these spots and en- 

 Fragment of an elementary fibre {from the Eel) par- o-asres Others, the ends Continue 

 tially contracted in water. Magnified 300 diameters. ~ 



a. Uncontracted part. b. Contracted part, along the to approximate, the parts OUCC 

 border of which the sarcolemma is raised from the sur- 

 face by the water that has been absorbed, and has contracted remaining; SO, be- 

 thereby caused the contraction, and by it has been 

 expelled from the contractile mass. Cause there IS UO force to CX- 



tend them. Hence the contraction appears permanent. 



In the latter case, certain parts of the fibre (as its broken extre- 

 mities) are fixed more or less firmly, so as to offer a resistance to the 

 contraction that takes place; this resistance enabling the contractile 

 force advancing to new parts to obliterate the traces of contraction 

 in the parts in which it is subsiding, 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 lamina of mica or glass with which it is requisite to cover the 

 object ; and they are the first to contract, because irritated by being 

 broken, and 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 approxima- 

 tion of the transverse stripes. These contractions 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 mu 



., 



