462 PRINCIPLES OF GENERAL PHYSIOLOGY 



one of a more physical nature. The nature of its energy as that of surfaces is also 

 confirmed by the fact that it has a negative temperature coefficient, while all 

 other possible forms of energy involved in muscular contraction have a positive one. 



The muscular system is analogous to that of a gas engine used to compress air 

 into a reservoir, from which it is taken to drive, by its pressure, various machines 

 and tools. The energy of the oxidation of the fuel is not used from the engine 

 directly. 



There is reason to believe that it is the lactic acid or its hydrogen ions that 

 is responsible for the changes in surface energy. 



The question of the origin of the lactic acid required to replace that lost to the 

 blood in vigorous muscular exercise, owing to deficient oxygen supply, is not yet 

 decided. 



The "efficiency" of the first, contractile, phase is practically 100 per cent., 

 that is, the whole of the tension developed can be used for work. That of the 

 whole process only amounts to about 50 per cent., since part of the chemical 

 energy of the oxidation process of the restoration period is lost as heat. 



The efficiency of the act of maintaining tension, as by holding up a weight, 

 is much less. This fact renders the calculation of the efficiency of the 

 whole animal or of an isolated organ, such as the heart, a matter of difficulty. 



The heart muscle shows particularly well certain characteristics which are 

 known to apply to all muscle, and others which probably apply. These phenomena 

 are : "all or nothing" in respect of strength of stimulus; "staircase," by which, 

 after a rest, successive contractions increase in height for a time, and probably 

 due to increase of hydrogen ions to the optimum point ; " refractory period," 

 as already described for nerve ; in certain conditions, summation of contraction ; 

 and great sensibility to certain ions, especially those of hydrogen. 



There is no transmission from fibre to fibre in skeletal muscle. It does, however, 

 take place in smooth muscle. It is not certain whether, in all cases, the spread 

 of excitation in all directions is conveyed by an intramuscular nerve network, 

 although it appears to be done by that of the Medusa. 



In warm-blooded animals, heat produced in muscular contraction is utilised 

 for the purpose of keeping up the temperature. The production of heat is 

 measured experimentally by specially constructed calorimeters, which also allow 

 the respiratory metabolism to be determined simultaneously. 



In adult animals at rest the production of heat is proportional to the external 

 surface, that is, to the loss. In work, since the muscles are producing heat in 

 excess, it approximates more to proportionality to weight. 



The temperature is regulated either by change of production, that is, by 

 greater activity or rest, or by change of loss, as by cutaneous vascular changes 

 and by evaporation of water in sweat and expired air. 



The co-ordinating centre of these factors of regulation is in the corpus striatum, 

 and is so arranged that it is sensitive to changes of temperature in the blood. 

 When warmed, this centre responds by causing muscular relaxation and dilatation 

 of skin vessels, thus producing a fall in body temperature. Conversely, when 

 cooled, it causes a rise in body temperature by exciting muscles to shivering 

 and by vascular constriction in the skin. It is probably also sensitive to afferent 

 impulses from heat and cold receptors in the skin. 



A certain degree of control of heat production appears to be the earliest form 

 of regulation, and is present in a rudimentary form even in cold-blooded animals. 

 As far up as Echidna, there is no other mechanism. 



The possible modes of origin of rhythmical contraction are discussed in 

 the text. 



In plants, movements are produced by changes of turgor, due to changes of 



