322 THE MUSCULATURE 



In the fully contracted condition the head and spinal column are strongly flexed, and the 

 digits are adducted, the various segments of the limbs are flexed and the limbs are adducted, 

 flexed and rotated medial ward toward the middle of the trunk. The body approaches a ball 

 in form. It is the position taken by a gymnast when turning a somerset in the air, and is in 

 marked contrast to the fully expanded condition which would be assumed could man fly like a 

 bat or gUde hke a flying squirrel. 



In general, the muscles which tend to put the body or a part of the body into the 

 expanded position form a distinct group as contrasted with those which tend to put the body 

 into the contracted position. The chief musculature which extends the head and trunk Ues 

 dorso-lateral to the spinal column and is supplied by the dorsal divisions of the spinal nerves. 

 The chief musculature which flexes the head and trunk lies ventro-lateral to the spinal column 

 and is supplied by ventro-lateral divisions of the spinal nerves. The chief muscles which 

 abduct, extend and rotate the limbs lateralward arise during embryonic development on the 

 dorsal sides of the limb buds and are innervated by branches from the dorsal sides of the brachial 

 and lumbo-sacral nerve plexus. The chief muscles which flex, adduct and rotate the hmbs 

 medialward arise on the ventral sides of the Umb buds and are supphed by nerves which arise 

 from the ventral sides of the hmb plexuses. To these general rules there are some exceptions, 

 the most marked of which is at the hip-joint where rotation of the girdle has brought about a 

 condition in which the primitive action of the flexor and extensor groups is partly reversed. The 

 chief flexors (the ilio-psoas and rectus femoris) belong to the dorsal division and some of the chief 

 extensors (the hamstring muscles) belong to the ventral division. At the ankle-joint the ex- 

 ception is more apparent than real. What is usually called flexion at the ankle-joint is really 

 hyper-extension and the reverse movement is the nearest we can come to real flexion. In 

 the extremely contracted position of the body as a whole the feet are extended (flexed plantar- 

 ward) at the ankle-joint. 



Muscles which produce a movement in a common direction are called 

 synergists, while those whose contraction produces opposite movements are 

 called antagonists; e. g., the flexors and extensors are antagonists. In the 

 actual working of the muscular system, however, when a set of muscles is 

 contracting to produce a movement, the antagonists also contract to a certain 

 degree. The movement is the result of nerve impulses sent simultaneously to 

 all the muscles which act on the part moved. l* 



The relation of the internal architecture of a muscle to the movements to which its contrac- 

 tion gives rise is a complex subject, the details of which cannot be entered into here. In general 

 it may be said that when the fibre-bundles run directly from one attachment to the other, as in 

 fig. 338, a and f, the force exerted by the contraction of the individual muscle-fibres is most 

 efficiently utihsed and the extent of the movement varies directly as the length of the fibres, 

 while the force exerted varies directly with the number of the fibres. 



In muscles of the types indicated in fig. 338, g, h, i, a certain amount of the extent of move- 

 ment and of the force exerted by the contraction of the individual fibres is not effectively 

 exerted on the parts moved by the muscles, as may be seen by applying to this action the laws 

 of the parallelogram of forces. In such muscles, however, the great number of short muscle- 

 fibres composing them makes possible the exertion of great power with some loss of speed of 

 contraction in the muscle as a whole. 



The direction of the movements which result from muscular contraction is in large part 

 determined by the shape of the articular surfaces, none of which are to be looked upon as simple 

 fulcra, but instead, during a given movement, the fulcrum shifts from one region to another of 

 the joint. 



In different muscles the extent of contraction of the constituent fibre-bundles during 

 activity varies considerably. While usually the length of the contracted fibre-bundles is 

 half that of those in the extended state, the amount of shortening in some muscles is only 25 

 to 35 per cent. 



Functional activity is necessary for the full development or for the maintenance of develop- 

 ment in muscles. Muscles atrophy if their nerve supply is injured or if they are passively 

 prevented from contracting. 



Order of treatment. — The muscles and fascia? are here treated in the following 

 order :■ — ( 1 ) those of the head and neck and shoulder girdle (p. 323) ; (2) those of the 

 upper extremity (p. 360); (3) those of the hack (p. 410); (4) those of the thorax 

 and abdomen (p. 422); (5) those of the pelvic outlet (p. 439); (6) those of the 

 lower extremity (p. 452). The reason for taking up the musculature in the order 

 named is, that during embryonic development musculature belonging primitively 

 to the head comes to overlap that of the neck; that of the neck spreads over the 

 region of the back and thorax, and becomes attached to the shoulder girdle; 

 that of the arm extends over the region of the thorax, a))domen, and back; that 

 of the back partially over the region of the thorax; while that of the abdomen 

 enters into intimate relation with the pelvic girdle. Ho far as practicable the 

 musculature of these various regions will be taken up according to fundamental 

 morpliologica! relationshijjs. 



Since a morphological grouping of the muscles does not accord perfectly with 

 a physiological grouping, there is given at the end of this section a table showing 

 what muscles are concerned in performing the simpler voluntary movements. 



