MUSCULAR ACTION 391 



on its back with a dead body in the same position, the absence of the 

 tonus in the latter case is obvious enough, and in other bodily positions is 

 still more striking. It is in part then by the tonus of the muscles that 

 posture is maintained. 



The central nervous system is probably the organ in which the co- 

 ordination of muscle-tonus is brought about. How the body's intention 

 to maintain a certain posture is connected with the nerves we have no 

 hint, and here we do not need to inquire. With little doubt the co- 

 ordinated tonus is maintained in most postures by the streaming of very 

 numerous mild impulses from the muscles' nerve-centers into the muscle- 

 bundles. These do not occasion a full twitch of the muscles, but only 

 just enough contraction to maintain, so to say, the "status quo." These 

 impulses are of somewhat the same nature as those which increase 

 muscular metabolism without causing any contraction, spoken of in our 

 discussion of thermotaxis. They are doubtless, however, of greater 

 intensity than the latter, just as these in turn are of less intensity than the 

 influences which bring about active molar contraction. 



It is interesting to observe how widely the intensity of contraction, if 

 not of its stimuli, must vary to maintain a posture under the many vary- 

 ing conditions of resistance and muscular vigor. Either of these mechan- 

 ical conditions in any case may vary within^ wide limits, but the nervous 

 system must and normally does see to it that just the right degree of 

 contraction is maintained to preserve the bodily position assumed. 



In some cases the maintenance of posture does not rest largely with 

 the muscles, for the bones bear the weight. By this means considerable 

 needless work is taken from the actively metabolic muscles and assumed 

 by the tissues (largely bones, tendons, aponeuroses, cartilages), that serve 

 more or less in a passive way. It is the work of the muscles in these cases 

 to pose the various parts of the body so that they are in equilibrium, and 

 to keep them thus, while the strain comes largely on the bones. In this 

 way only a minimum of exertion is demanded of the muscles. Perhaps 

 the most important illustration of this is in standing. 



STANDING. It was formerly thought that the muscles were largely 

 responsible directly for the maintenance of the erect position. The 

 work of Braune and Fischer, v. Meyer, and others shows that the body's 

 equilibrium is almost wholly a passive affair so long as the joint-centers 

 are kept in the "normal position" and the muscle-tonus maintained. 

 In this exact posture the center of gravity of the body and all the joint- 

 centers are in line. The knee-joints, hip-joints, the atlo-axoid, and the 

 ankle-joints lie in the same vertical plane in which are also the centers of 

 gravity of the head, trunk, thighs, legs, upper arms, forearms, and hands. 

 Hence in normal standing each of these parts and joints is in balance 

 and also the whole body together. Braune and Fischer found that the 

 knee-joint's mid-point was 4 cm., the hip-joint's 5 cm., and the atlo- 

 axoid's 3.5 c.m. in front of the vertical plane passing through the ankle- 

 joint's mid-points, while the center of gravity of the head was 4 cm. and 

 of the whole body 4.2 cm. in front of this plane. The center of gravity 



