ACTIONS OF MUSCLES PASSING FROM THE TRUNK. 213 



levator scapulae, the external intercostals, or tlie external oblique. With the levator scapulae 

 it forms one muscle in many mammals. 



Nerves. The nerves which supply the anterior muscles passing from the trunk to the 

 upper limb are all derived from the brachial plexus. The nerve of the subclavius is a small 

 twig- from the trunk formed by the fifth and sixth cervical nerves. The large nerve of the 

 serratus magnus, called posterior thoracic, proceeds from the fifth, sixth, and generally also 

 the seventh nerves : the upper division of the muscle is supplied from the fifth nerve ; the 

 middle division from the sixth, sometimes also the fifth ; and the lower division from the 

 sixth and seventh. The nerves of the pectoral muscles are the two anterior thoracics from 

 the inner and outer cords of the plexus : the clavicular part of the pectoralis major is sup- 

 plied from the fifth and sixth cervical, the sterno-costal from the lower four cervical and the 

 first dorsal nerves, the upper fasciculi from the higher ,nerve and rice versa : the pectoralis 

 minor receives its fibres from the seventh and eighth cervical and first dorsal nerves. (On 

 the nerves supplying the muscles of the upper extremity, see W. P. Herringham, Proc. Roy. 

 Soc., xli. 423.) 



Actions of the muscles passing- between the trunk and upper limb. Considered with 

 reference to the movements of the limb upon the trunk, it is to be observed that the muscles 

 passing to the scapula are so disposed that each one, while drawing that bone in a definite 

 direction over the thorax, also tends to impart to it a movement of rotation, and that in order 

 to produce the simple gliding movements, upwards and downwards, forwards and backwards, 

 various combinations are necessary, as is explained in detail below. 



The superior fibres of the trapezius elevate the clavicle and scapula ; the middle fibres 

 acting on the acromion have also some elevating action, but tend rather to carry back the 

 scapula towards the spine ; the inferior part of the muscle acting upon the spine of the 

 scapula would of itself depress that bone while it carries it inwards towards the dorsal spines, 

 but acting in concert with the upper two-thirds of the muscle, a rotation is produced in the 

 scapula round a central point, in such a manner that while the acromion is somewhat raised 

 and carried towards the dorsal spines, the upper angle of the scapula is depressed, and the 

 lower angle is carried outwards and elevated. 



The levator anyuli scapula and r/tomboidei elevate the superior angle and base of the 

 scapula, thus counteracting the rotating action of the trapezius. By their action, in combina- 

 tion with the upper fibres of the trapezius, the scapula is raised without rotation, while the 

 middle and lower portions of the trapezius with the rhomboidei draw the scapula backwards 

 and inwards, towards the dors*al spines. The rhomboidei muscles are the special antagonists 

 of the serratus magnus. 



The subnlavivs depresses the clavicle, and may also act as a support to the sterno-clavicular 

 articulation. 



The pert or alls minor draws the coracoid process downwards and forwards, and tends to 

 throw the lower angle of the scapula backwards. It acts with the serratus magnus in drawing 

 forwards the scapula, with the subclavius and lower part of the trapezius in depressing the 

 shoulder, and with the levator and rhomboidei in producing downward rotation of the scapula 

 when the arm is brought to the side. 



The xcrrutux mafjnus muscle, by withdrawing the base of the scapula from the spinal 

 column, enables the arm when raised from the shoulder to be still farther outstretched, as in 

 the movement termed extension in fencing : it comes powerfully into action in all movements 

 of pushing. Its lower portion likewise combines with the trapezius in rotating the scapula 

 upwards in elevation of the arm. 



The latis.nmus dorsi carries the elevated arm downwards and backwards, rotating it at the 

 same time inwards. Acting through the humerus, it also assists in drawing the shoulder 

 downwards and backwards. 



The pectoralis major, while it combines with the latissimus dorsi in depressing the humerus 

 from the abducted position, opposes that muscle by drawing the limb forwards. The lower 

 part of the muscle acts most efficiently in adducting, the succeeding fasciculi draw the arm 

 more forwards, and the clavicular portion is mainly a flexor of the shoulder- joint, raising the 

 arm in a direction forwards and inwards. 



Considered as acting on the trunk from the upper limb in a fixed condition, these muscles 

 all tend to draw the trunk of the body towards the limb, as in climbing, or other like efforts. 

 The latissimus dorsi muscles carry the body upwards and forwards, as in the use of crutches. 

 The pectorales are also muscles of forced inspiration, tending to raise the ribs, more especially 

 when the shoulders are fixed and the arms are elevated ; and it is thought by some that the 

 lowest slips of the serratus magnus may have a similar action when the scapula is fixed by 

 the muscles passing to it from the spine posteriorly and superiorly. The upper parts of both 

 trapezii acting on the occipital bone aid in extending the head on the vertebral column ; and 

 if one muscle only acts, it aids in rotating the head. 



p 2 



