830 OF THE STRUCTURE AND FUNCTIONS OF MUSCULAR TISSUE. 



ate with those of others, we find that such a method of proceeding affords 

 no assistance whatever, but rather tends to impede our progress, by drawing 

 off the attention from the " guiding sensations" (visual, muscular, etc.), which 

 are the only regulators that can be depended upon for determining the 

 due performance of the volitional mandate. Hence we are led by these con- 

 siderations, as by those stated in the preceding paragraph, to the conclusion 

 that the agency w : hich directly affects the muscles is of the same kind, and 

 that it operates under the same instrumental conditions whatever be the 

 primal source of the motor power. And in watching the gradual acquire- 

 ment of the capacity for different kinds of movement, during the periods of 

 Infancy and Childhood in the Human subject, we find everything to confirm 

 this conclusion. For it becomes obvious that the acquirement of Voluntary 

 power over the movements of the limbs, is just as gradual as it is over the 

 direction of the thoughts; all the activity of the body, as well as of the mi ml, 

 being in the first instance automatic, and the Will progressively extending 

 its domination over the former, as over the latter, until it brings under its 

 control all those muscular movements which are not immediately required 

 for the conservation of the body, and turns them to its own uses. 1 



2. Of the Symmetry and Harmony of Muscular Movements. 



680. It might have been not unreasonably supposed, a priori, that those 

 muscles would have been most readily put to simultaneous contraction which 

 correspond to each other on the two sides of the body ; in other words, that 

 symmetrical movements would be those most readily performed. Such, how- 

 ever, is by no means the case; for in many of our most familiar actions we 

 consentaneously exert different muscles on the two sides of the body. This is 

 nowhere more clearly shown than in the various movements that are re- 

 quired for the performance of the different acts of locomotion, and which 

 may here be briefly noticed. In order to maintain the body in the erect 

 posture, simple as the effort appears, the concurrent action of many muscles 

 is required, as is clearly shown not only by the numerous and futile trials 

 made by children before the power of balancing is acquired, but by the im- 

 possibility of placing a dead body in this position without support. In 

 standing the legs are more or less extended ; and the weight of the trunk is 

 transmitted through the femora and the tibise and fibula? to the astragali. 

 These, with the other tarsal and metatarsal bones, form an elastic arch on 

 either side, which, whilst allowing the great superincumbent pressure to be 

 borne with ease, enables each foot to accommodate itself to irregularities of 

 the surface of the ground, and at the same time breaks the shock or jar 

 which would otherwise be experienced in the various movements of walking, 



1 The aptitude which is acquired by practice, for the performance of certain ac- 

 tions that were at first accomplished with difficulty, seems to result as much from a 

 structural change which the continual repetition of them occasions in the Muscle, as 

 in the habit which the Nervous system acquires of exciting the movement. Thus 

 almost every person learning to play on a musical instrument finds a difficulty in 

 causing the two shorter fingers to move independently of each other and of the rest; 

 this is particularly the case in regard to the ring finger. Any one may satisfy him- 

 self of the difficulty by laying the palm of the hand flat on a table, and raising one 

 tinker after the other, when it will he found that the ring finger can scarcely be lifted 

 without disturbing the rest, evidently from the difficulty of detaching the action 

 of the portion of Ihe extensor communis digittn-iun, by which the movement is pro- 

 duced, from that, of the remainder of the muscle. Yet to the practiced musician, 

 the command of thu Will over all the fingers becomes nearly alike; and it can 

 scarcely he doubted that some change in the structure of the muscle, or a new devel- 

 opment of its nerve-fibres takes place, which favors the isolated operation of its sev- 

 eral divisions. 





