246 



GENERAL PHYSIOLOGY 



IS 



mma 



substance flows into these tubes, so that the lumen of each 

 enlarged and the whole segment becomes broader and shorter. 

 All these complex phenomena of contraction proceed with ex- 

 cessive rapidity from one muscle-segment to the following, so that 

 one contraction-wave after another passes metachronically over all 

 the elements of the whole muscle-fibre until the latter is completely 

 contracted. The expansion of smooth and cross-striated muscle-fibres 

 shows exactly the reverse of all these events observed during the 

 contraction. The fibrillse extend, becoming gradually longer and 

 thinner from the point where the contraction-wave previously 

 began, so that now a wave of expansion proceeds from here over 

 the whole fibrilla, until the latter is completely extended. In the 

 single segment of the cross-striated fibre, 

 also, the changes are exactly the reverse 

 of those that appear in contraction. The 

 segment becomes longer and thinner, the 

 anisotropic substance decreases in volume, 

 and becomes darker, denser, and more highly 

 refracting, while the isotropic substance 

 gains in volume and becomes lighter, less 

 dense and less refracting, until the resting 

 state is again reached. In the expansion 

 of the cross-striated muscle-fibre, therefore, 

 substance that possesses slight consistency passes 

 from the anisotropic to the isotropic discs. 



Both smooth and cross-striated muscle- 

 fibres are united in the cell-community 

 into tissues, the muscles. Wherever rapid and powerful mus- 

 cular effects are to be brought about, as in the skeletal 

 muscles and the heart, the muscles are composed of cross-striated 

 fibres, while the slow, sluggish movements of the involuntary 

 organs, such as the stomach, the intestine, and the bladder, 

 depend upon the activity of smooth muscle-cells. The contraction 

 of miiscle reaches its highest, and indeed, an astonishing rate in 

 the wing-muscles of many insects, e.g., gnats, where, as Marey has 

 shown, 300 — 400 contractions in a second can be carried out. It 

 is evident that the effect of the contractions will be very consider- 

 able where very many fibres compose a muscle. In fact, even in 

 relatively small muscles an enormous transformation of energy 

 takes place. Thus, such a small muscle as the calf-muscle (gastroc- 

 nemius) of the frog, which measures scarcely a centimetre in cross- 

 section in its thickest place, is capable, according to Rosenthal's 

 observations, of raising a weight of more than one kilogram. The 

 work that the heart-muscle performs is enormous. Zuntz ('92) has 

 calculated that the heart of a man beating normally performs in 

 one day a work of about 20,000 kilogram-metres — a labour that 

 would be sufficient to raise a weight of 20,000 kilograms one metre 



Fig. 106. — Muscle-segments 

 from the wasp, containing 

 tubes of anisotropic sub- 

 stance ; a, anisotropic sub- 

 stance seen from above ; b, 

 from the side ; c, three 

 muscle-segments. (After 

 Schafer.) 



