8 June, 1907.] lUciuculs of Aiiiiual Plixsiology. 343 



and the other flexor. The necessity of such an arrangement is obvious, 

 for a muscle, whilst it can pull cannot push, and furthermore delicate and 

 skilled movement of a limb can be best brought about by having both sets 

 contracted and allowing one set to act a little more powerfully than the 

 other. In ball and socket joints such as the hip, shoulder and eye we 

 find sets of muscles on four or more aspects so that the range of move- 

 ment may be increased. 



Nature of muscle contraction. — A muscle is in realitv an engine 

 for transforming energ\ into work. Each muscle is continuallv drawing 

 on the blood with which it is copiouslv ]:)erfused for a supply of nitrogen- 

 free carbon compounds with a relatively high potential energy. When 

 the muscle contracts these compounds are broken down and energv is 

 liberated. A sufficient supply of oxygen is also essential. It was for- 

 merly thought that these carbon compounds were burned by the oxvgen 

 with the formation of heat and that the heat thus produced made the 

 prisms in the muscle cell contract, just as stretched catgut will contract 

 when its temperature is raised. But muscle works far too economicallv 

 for this to be the case. More probably the carbon compounds, bv a 

 partial disintegration, give rise to electric energy which is transformed 

 into work ; then these break down products, which if allowed to accumu- 

 late would check the production of electric energy, are burnt up by the 

 ox\gen. Whatever be the exact nature of the transformation, we know 

 that the final chemical products of muscular activitv are water and car- 

 bonic acid. If oxygen be withheld from the muscle the latter can go 

 on contracting for some time but weaker and weaker until it eventual) \ 

 ceases to respond to even the most urgent ner\e impulses. If we examine 

 a mu-scle in this condition we shall find that it is acid in reaction, and that 

 this acidity is due to the presence of lactic acid which possibly represents 

 one of the break down products which normally are burnt off. 



The carbon compounds which bring ]X)tential energy to muscle can be 

 derived from either carbohvdrates, proteins or fats. In the case of pro- 

 teins the nitrogenous parts 0:6 the molecule are previously split off, forming 

 urea which leaves the body by the kidn^^y, whilst the more carbonaceous 

 part can be used as a muscle fuel-food. There is some evidence to show 

 that the carbon compounds derived from proteins can yield their energ\- 

 to muscle at a quicker rate than those derived from carbohvdrate and fats. 

 An ox, for instance, whose diet is largely carbohydrate, may do far more 

 work in a dav than a tiger, but such work is performed in a com- 

 paratively slow and regular manner. The tiger, as a protein-eater, can 

 command a sudden liberation of energy in its muscles and can in conse- 

 quence drag the ox clown, kill it, and run off with it. The same thing 

 is shown in horses which are put on a liberal protein ration ; they become 

 spirited and fresh and are better able to produce the swift and p'owerful 

 muscular contractions that are needed in a race. 



It has been stated that a muscle as an engine works eronomically. 

 Speaking generally a mu.scle can transform 30 per cent, of the energy it 

 receives into work, the remaining 70 per cent, taking the form of heat 

 which warms the muscle and escapes into the blood and surrounding 

 tissues. Another way of stating this fact is to say that muscles supplied 

 with a certain amount of carbonaceous matter will produce only 70 per 

 cent, of the heat which these same compounds would give rise to if burned 

 elsewhere. Now this 30 per cent, efficiency of a muscle places it far 

 above any steam engine as an economical machine. It is not however 



