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SIXTH LAYER. 



Actions. The upper fibres of the trapezius draw the shoulder up- 

 wards and backwards ; the middle fibres, directly backwards ; and 

 the lower, downwards and backwards. The lower fibres also act 

 by producing rotation of the scapula upon the chest. If the shoulder 

 be fixed the upper fibres will flex the spine towards the correspond- 

 ing side. The latissimus dorsi is a muscle of the arm, drawing it 

 backwards and downwards, and at the same time rotating it in- 

 wards; if the arm be fixed, the latissimus dorsi will draw the spine 

 to that side, and raising the lower ribs be an inspiratory muscle ; 

 and if both arms be fixed, the two muscles will draw the whole 

 trunk forwards, as in climbing or walking on crutches. The levator 

 anguli scapulae lifts the upper angle of the scapula, and with it the 

 entire shoulder, and the rhomboidei carry the scapula and shoulder 

 upwards and backwards. 



The serrati are respiratory muscles acting in opposition to each 

 other the serratus posticus superior drawing the ribs upwards, and 

 thereby expanding the chest ; and the inferior, drawing the lower 

 ribs downwards and diminishing the cavity of the chest. The former 

 is an inspiratory, the latter an expiratory muscle. The splenii 

 muscles of one side draw the vertebral column backwards and to 

 one side, and rotate the head towards the corresponding shoulder. 

 The muscles of opposite sides acting together, will draw the head 

 directly backwards. They are the natural antagonists of the sterno- 

 mastoid muscles. 



The sacro-lumbalis with its accessory muscle, the longissimus dorsi 

 and spinalis dorsi, are known by the general term of erector spines, 

 which sufficiently expresses their actions. They keep the spine 

 supported in the vertical position by their broad origin from below, 

 and by means of their insertion by distinct tendons into the ribs and 

 spinous processes. Being made up of a number of distinct fasciculi, 

 which alternate in their actions, the spine is kept erect without 

 fatigue, even when they have to counterbalance a corpulent abdomi- 

 nal developement. The continuations upwards of these muscles into 

 the neck preserve the steadiness and uprightness of that region. 

 When the muscles of one side act alone, the neck is rotated upon 

 its axis. The complexus, by being attached to the occipital bone, 

 draws the head backwards, and counteracts the muscles on the 

 anterior part of the neck. It assists also in the rotation of the head. 



The semi-spinales and multifidus spines, muscles act directly on 

 the vertebras, and contribute to the general action of supporting the 

 vertebral column erect. 



The four little muscles situated between the occiput and the two 

 first vertebra?, effect the various movements between these bones ; 

 the recti producing the antero-posterior actions, and the obliqui the 

 rotary motions of the atlas on the axis. 



The actions of the remaining muscles of the spine, the supra and 

 inter-spinales and inter-transversales, are expressed in their names. 

 They approximate their attachments and assist the more powerful 

 muscles in preserving the erect position of the body. 



