THE DISSECTION OF THE BACK 77 



to the opposite side. It is supplied by the posterior ramus 

 of the sub-occipital nerve (Fig. 20). 



The Actions of the Deep Muscles of the Back. The 

 dissector will have noted that many of the deep muscles of the 

 back, such as the various prolongations of the sacro-lumbalis, 

 run vertically upwards ; others run upwards and medially, 

 viz., the semispinalis dorsi and cervicis and the multifidus 

 spinae. A third group, exemplified by the splenius capitis and 

 cervicis, the serratus posterior inferior and the inferior oblique 

 muscle, run upwards and laterally. When the muscles which 

 run vertically upwards contract, on one side only, they bend 

 the vertebral column to that side, but if the muscles of both 

 sides act simultaneously they bend the vertebral column 

 backwards. When the muscles which run upwards and 

 laterally contract they turn the head or trunk to the same 

 sides, whilst those which run upwards and medially turn the 

 head or trunk to the opposite side. The muscles which lie 

 at the sides of the sub-occipital space need further considera- 

 tion. They act either upon the occipito-atlantal joints, the 

 joints between the first and second cervical vertebrae, or 

 upon both sets of joints. At the occipito-atlantal joints back- 

 ward and forward movement and a slight oblique movement 

 whereby the head is turned a little to one or the other side 

 take place. The main movement between the atlas and the 

 second cervical vertebrae is a movement of rotation, the atlas 

 carrying the head rotating around the dens of the second 

 vertebrae. 



The rectus capitis posterior minor and the superior 

 oblique act on the joints between the atlas and occipital 

 bone only : the rectus capitus posterior minor producing 

 backward movement only and the superior oblique backward 

 movement and a very slight oblique movement which turns 

 the head slightly towards the opposite side, The inferior 

 oblique acts only on the joints between the atlas and the 

 second vertebra, turning the head to the same side. The 

 rectus capitis posterior major alone acts on both sets of 

 joints, drawing the head backwards and turning it to the 

 same side. 



Posterior Ramus of the Sub -Occipital Nerve. The 

 posterior ramus of the sub -occipital nerve does not divide 

 into medial and lateral divisions. It enters the sub-occipital 

 triangle by passing dorsally, between the posterior arch of 



