THE MUSCULAR SYSTEM 411 



to rise 5 F. through feeding. The whole body temperature is 

 raised during work, especially in the horse, and does not fall for 

 some time after. In dogs also a rise of temperature of several 

 degrees may be obtained by stimulating the spinal cord and 

 thus producing muscular contractions. A contracting muscle 

 liberates energy in the form of both work and heat. We have 

 seen reasons for regarding the oxidation of the non-nitrogenous 

 carbohydrates as the normal source of this energy, and have 

 referred to the quiescent storage of oxygen during rest as account- 

 ing for the prolonged possible activity of an isolated frog's muscle. 

 In connection with this it is not uninteresting to calculate, as 

 Fick has done, the amount of carbohydrate necessarily oxidised 

 to provide all the energy as work + heat furnished by a single 

 maximally vigorous contraction of a frog's muscle. Knowing 

 the heat of combustion of, say, glycogen, and converting the 

 work of the muscle into heat by Joule's equivalent, we find that 

 1 gramme of frog's muscle can provide all the energy it sets free 

 in a single maximal contraction by the oxidation of 0-0006 milli- 

 gramme of carbohydrate. In the case of fat the necessary 

 amount would be still less — viz., 0*00025 milligramme. This 

 may serve to diminish our surprise at the working activity of 

 which an excised muscle is capable. 



Causation of a Muscular Contraction. — This is a problem as yet 

 unsolved ; our previous studies in every way point to the oxidation 

 of carbohydrate substance as being the source of energy, and we 

 have seen that it is impossible for a muscle to contract without 

 using up oxygen and producing carbon dioxide. But we are now 

 brought face to face with a paradoxical condition ; if muscle be 

 exposed to the vacuum of a gas-pump no free oxygen can be 

 obtained from it, while if the ordinary nerve-muscle preparation 

 be taken and placed in a jar of hydrogen it continues to contract, 

 and, even still more remarkable, it continues to produce C0 2 

 though no oxygen exists in the atmosphere surrounding it. As 

 C0 2 cannot be formed without oxygen, it is evident the oxygen 

 must come out of the muscle, and to meet this difficulty it is 

 supposed that the muscle molecules store up oxygen during rest 

 in a hypothetical compound of hydrogen and carbon, to which 

 the name inogen has been given ; during muscular contraction 

 this compound breaks down, and the waste products are liberated. 



Statements such as these help very little to a better under- 

 standing of the cause of a muscular contraction. No one doubts 

 the process being at bottom chemical in its nature, large and 

 complex molecules being reduced to smaller and simpler ones, 

 but how this can lead to contraction of a muscle is still un- 

 explained. An impulse is conveyed to a muscle by means of a 

 motor nerve, and through the medium of an end-plate it becomes. 



