. A manual of diseases of the nervous system. raises it. The muscle is usuallyparalysed with the trapezius, and then the scapula falls, but the special effectof its palsy is lost in that of the trapezius. If the levator is preserved, and thetrapezius paralysed, there is great rotation of the scapula, which is, as it were,suspended by its inner angle (Fig. 7). Serratus magnus (posterior thoracic nerve—C. 5 and 6) carries the scapulaoutwards, forwards, and slightly upwards when the arm is put forwards. Ittends to rotate the scapula on the inner angle (acromion up), the lower fibresmost powerfull
. A manual of diseases of the nervous system. raises it. The muscle is usuallyparalysed with the trapezius, and then the scapula falls, but the special effectof its palsy is lost in that of the trapezius. If the levator is preserved, and thetrapezius paralysed, there is great rotation of the scapula, which is, as it were,suspended by its inner angle (Fig. 7). Serratus magnus (posterior thoracic nerve—C. 5 and 6) carries the scapulaoutwards, forwards, and slightly upwards when the arm is put forwards. Ittends to rotate the scapula on the inner angle (acromion up), the lower fibresmost powerfully, hut this rotation is prevented by the rhomboids and levatoranguli. It does not raise the shoulder when the arm is hanging. It helps tofix the scapula when the posterior fibres of the deltoid move the raised armback. If the scapula is fixed by the rhomboids, the serratus can act on theribs, and aid foiced inspiration. It has most inspiratory effect when the armsare elevated. In paralysis there may be little change in the position of the. trapezius; rotation ofscapula, from weight ofarm, in consequence ofthe paralysis of the tra-pezius; progressive mui«cnlar atrophy. 36 ACTION AND PARALYSIS OF MUSCLES. scapula at rest, but often there is slight rotation (lower angle in) from theunopposed tone of the rhomboids. When the arm is moved forwards by theanterior part of the deltoid, the scapula, no longer held against the thorax andmoved forwards by the serratus, is rotated on its vertical axis by the action ofthe anterior part of the deltoid on the humerus, and of the middle part on thescapula. Thus the posterior edge recedes from the thorax, leaving a grooveinto which the hand can sometimes be placed (Fig. 8). The scapula is at thesame time rotated, lower angle inwards and upwards. Elevation of the armabove the level of the shoulder is much weakened, but can be imperfectlyeffected by the middle part of the trapezius. Loss of the serratus weakensother movements, but does not abol
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