CONTRACTILE TISSUES 455 



closed by a strong, slowly acting muscle, which follows up, as it were, the rapid 

 contraction of the other and holds the shells together. This last effect seems to be 

 done by some kind of an arrangement which may be compared to a ratchet, the 

 process being attended with no consumption of energy. A description of certain 

 of these mechanisms will be found in the book by von Uexkilll (1909, pp. 92 and 

 144). The question is discussed in Chapter XVIII. 



TRANSMISSION OF EXCITATION IN MUSCLE 



In skeletal muscle it is important for sensitive grading of contraction that the 

 fibres should act separately. In smooth muscle and in the heart, excitation can 

 travel from one fibre or cell to another, so that there are waves of contraction 

 passing over the mass of muscle. In the heart, the separate fibres are connected 

 by bridges of muscular structure, but, in the typical smooth muscle, such as the 

 intestine, it is more difficult to ascertain the mode of transmission from cell to cell. 

 In both cases, however, it can be seen that a stimulus applied to a point starts 

 a wave of contraction, which travels in all directions from the point stimulated. 



It is very instructive to lead off the quiescent ventricle of the frog, or better of the tortoise, 

 by two electrodes to a capillary electrometer, the electrodes being as far apart as possible, say 

 on the apex and base, respectively. If an induction shock is applied close to one of the 

 electrodes, say that at the base, a diphasic electrical response will be seen, indicating by 

 its direction that a negative wave has started at the base and been propagated to the apex. 

 The neighbourhood of the electrode at the apex is then stimulated ; we see again a diphasic 

 response, but this time the negative wave starts at the apex and is propagated to the base, 

 so that, if the first phase in the first experiment was an upward movement of the mercury, 

 in the second experiment it will be downward. 



It is not to be taken for granted that the muscular systems of the lower 

 invertebrates necessarily behave in the same way as the smooth muscle of the 

 vertebrate. It is important for their movements that accurate control should be 

 exercised over separate fibres and, accordingly, we find that (von Uexkiill, 1909, 

 p. 79), even in the Medusae, the contraction produced by an electrical stimulus 

 remains, in certain cases, limited to the spot excited ; the excitatory process does not 

 spread from one fibre to another. This applies to the ring of muscle around the edge 

 of the umbrella of Rhizostoma. On the other hand, in Aurelia, as Romanes has 

 shown (1876), the umbrella can be cut up into a spiral or other shape, and a con- 

 traction produced by stimulus applied at one end is conducted to the other end. 

 Romanes (1885, p. 77), however, regards it as proved that the excitatory process is 

 conveyed by the nerve network and not by transmission from muscle cell to 

 muscle cell directly. The properties of such nerve networks will be discussed in 

 the next chapter. 



PRODUCTION OF HEAT 



The time relations of the production of heat in muscular contraction have 

 been described above. We saw that, in the restitution process, or reaction by 

 which lactic acid is restored to its place in the system, which is thus given a store 

 of potential energy, a certain amount of chemical energy is degraded to heat, and 

 also that the contractile tension developed on excitation, if unused for the 

 performance of external work, is transformed to heat in the muscle itself. 



Now, in the warm-blooded animal, this heat must not be looked upon as entirely 

 wasted, since it serves to keep up the temperature of the organism. The import- 

 ance of this raised temperature for the hastening of chemical reactions, in response 

 to changes in the environment, has been pointed out. 



Muscular contraction is, in fact, the chief, if not the only, source of heat 

 of practical importance to the animal organism. Of course, heat is produced 

 in other chemical reactions, especially those of oxidation, but they make up 

 but a small part of the total. 



Even cold-blooded animals and plants produce heat, but they are not provided 

 with arrangements for keeping their temperature constant, so that it is usually 

 only a fraction of a degree higher than that of their surroundings. Hence they 

 are called " poikilothermic," of varying, temperature, whereas the higher 



