THE MUSCLES AND OTHER ORGANS OF THE BODY 449 



characteristic properties. On the other hand, a muscle increases in mass 

 by work, and there is, generally speaking, no other means of strengthening 

 a muscle. We see therefore that a muscle receives impulses from the central 

 nervous system which are of the greatest possible importance for the mainte- 

 nance of its substance and of its natural properties (see Chapter XXII). 



A resting muscle has a relatively small supply of Uood, but during work 

 the quantity increases considerably, owing to the widening of the blood stream 

 produced by the action of the vasodilator nerves (cf. page 240). Besides, 

 we find as an accompaniment of muscular work an acceleration of the heart 

 beat (cf. page 197) and, as a rule, an increase of arterial blood pressure. The 

 latter is caused primarily by a contraction of blood vessels in other organs, 

 especially those of the splanchnic region, which more than compensates for 

 the dilation in the muscles. The increase in amount of blood expelled from 

 the heart in a unit of time likewise contributes to the same end. 



It is impossible, on the basis of observations thus far recorded, to make 

 a closer analysis of the mechanisms concerned. 



Vaso dilatation in the muscles accompanying work is for the purpose of 

 supplying them with an increased amount of oxygen and combustible mate- 

 rials ; for a working muscle uses large quantities of oxygen and produces large 

 quantities of carbon dioxide. In order to supply the necessary quantity of 

 oxygen and to remove the great excess of carbon dioxide, the respiration must 

 of course be augmented, and this should be mentioned as one of the accom- 

 paniments of muscular work (cf. page 332). 



Muscular work evidently calls for an increased supply of food in order to 

 meet the demands on the body, and increased appetite as the result of exercise 

 is an experience with which everyone is familiar. 



Whatever the effect of work on the digestive process may be, it appears from 

 the experiments of Rosenberg on dogs and of Wait on men that the absorption 

 of food is equally good at rest and at moderately vigorous work. 



With all voluntary muscular movements work is being done also in the 

 central nervous system. When we learn a particular muscular movement, of 

 whatever kind, the brain is always active. The newborn child can move all 

 of his muscles, but lacks the power to coordinate them into purposeful acts. 

 This can only be acquired by the gradual formation of central connections 

 between the different nerve paths. We know, for example, that many muscles 

 are necessary to keep the body in an upright position, but the cooperation of 

 these different muscles is perfected only by long-continued practice. So it is 

 with all of the other muscular movements which we make. 



Unless we make a special study of the subject we are not aware of the 

 position or arrangement of our muscles. We cannot therefore merely will 

 that one muscle or the other shall become active but can only resolve upon 

 carrying out a certain movement. For example, if we bend the arm, the 

 movement takes place chiefly by the contraction of the biceps muscle; but 

 the act of volition, which we are conscious of, is not a direct impulse to this 

 particular muscle, but a command that the arm be moved. In short we carry 

 out our bodily movements with reference to the result, without troubling 

 ourselves about how the result is attained. 



