CHAPTER X 



THE ORGANS OF LOCOMOTION 



The muscular tissues of an animal have the special function 

 of altering the positions of various parts of the body relative to 

 one another. By means of this function, which is due to the 

 contractility of the tissue and its capacity to respond to nervous 

 impulses, locomotion is effected. The principal kinds of muscle 

 have been already sufficiently described. It has also been men- 

 tioned that whereas the muscles of the heart and blood vessels 

 and the unstriated inuscles generally are involuntary, those of 

 the limbs, which are striated, are under the control of the will. 



The contraction of a particular muscle (e.g. the gastrocnemius 

 of the calf of the frog's leg) can be caused experimentally by 

 stimulating electrically the nerve which supplies it (e.g. the 

 sciatic), and if the stimuli are made to succeed one another at 

 a rate such that the muscle has not begun to relax before the 

 next stimulus reaches it, it is driven into a condition of continuous 

 contraction or 'tetanus.' 



The process of contraction m muscle is associated with the 

 splitting off of lactic acid, but at first there is no evolution of 

 carbon dioxide. This takes place in the second stage which 

 succeeds the contraction stage. Fats or carbohydrates are then 

 oxidised, and the lactic acid is restored to the muscle. Energy 

 is set free equivalent to the heat evolved and the work done, 

 the former being utilised to keep up the temperature of the body. 



It is well known that exercise increases muscular efficiency. 

 This is due partly to the direct effect of exercise in developing 

 the muscles, and partly to the fact that with repeated usage the 

 muscles work more economically, those which are superfluous 

 remaining at rest. It is only after repeated trials that the 

 nervous impulses become properly distributed to the necessary 

 muscles. 



The composition of muscular tissue varies within considerable 

 limits owing to the inconstant quantities of fat or connective 



M. p. 8 



