SIXTH LAYER. 207 



wards and downwards, and at the same time rotating it inwards ; 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. 



In examining the following table, the student will observe the 

 constant recurrence of the number four in the origin and insertion of 

 the muscles. Sometimes the four occurs at the top or bottom of a 

 region of the spine, and frequently includes a part of two regions, and 

 takes two from each, as in the case of the serrati. Again, he will 

 perceive that the muscles of the upper half of the table take their 

 origin from spinous processes, and pass outwards to transverse, whereas 

 the lower half arise mostly from transverse processes. To the student, 

 then, we commit these reflections, and leave it to the peculiar tenor of 

 his own mind to make such arrangements as will be best retained by 

 his memory. 



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 erectores spines, 

 which sufficiently expresses their action. They keep the spine sup- 

 ported 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 abdominal 

 development. 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 

 vertebrae, and contribute to the general action of supporting the verte- 

 bral column erect. 



The four little muscles situated between the occiput and the two first 

 vertebrae, effect the various movements between these bones ; the recti 

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

 motions of the atlas on the axis. 



