THE BACK. 405 



Insertion. It is continuous abov r e with the spinalis dorsi, longissimus dorsi, 

 and ilio-costalis. 



Locate the spinalis dorsi first, the inner insertion of the erector spinae 

 muscle. Remember it takes its origin from the inner part of the erector spinae. 

 Trace its tendons of insertion to the spines of the upper thoracic vertebrae. 



Locate the groove between the outer and middle divisions of the erector 

 spinae. In this groove you will find the external branches of the posterior 

 divisions of the spinal nerves, in company with the dorsal branches of the inter- 

 costal vessels. Turn the muscles apart, as in figure 284. 



The outer division of the erector spinae is called ilio-COStalis, to its insertion 

 into the angles of the lower ribs, from the sixth to the eleventh rib. It is continued 

 upward through the back and neck as (i) the accessorius, (2) cervicalis ascendens. 



Accessorius ad ilio-costalem arises from the angles of the ribs, seventh to 

 eleventh, and is inserted into the angles of the ribs from the second to the fifth, 

 and into the transverse process of the seventh cervical vertebra. 



Cervicalis ascendens arises from the ribs fourth to fifth upper internal to 

 the costal insertion of the accessorius, and is inserted into the posterior tubercles 

 of the fourth, fifth, and sixth cervical transverse processes. 



The middle division of the erector spinae continues through the thorax under 

 the name of longissimus dorsi ; through the neck, to the head, as the trans- 

 versalis colli and trachelo-mastoid. (Fig. 284.) 



The longissimus dorsi arises: (i) From the middle part of the erector 

 spinae ; (2) from the transverse processes of some of the lower thoracic vertebras. 

 It is inserted into : (i) The ribs external to their tubercles ; (2) the transverse 

 processes of the thoracic vertebras ; (3) into the accessory tubercles of the upper 

 lumbar and lower thoracic vertebrae ; (4) into the transverse processes of the 

 upper lumbar vertebrae. 



Transversalis colli arises from the transverse processes of the upper six 

 thoracic vertebrae, internal to the insertion of the longissimus dorsi. It is in- 

 serted into the posterior tubercles of the transverse processes of the vertebrae from 

 the second to the sixth cervical vertebrae inclusive. 



The trachelo-mastoid (Fig. 284) is the inner part of the transversalis colli 

 continued in the mastoid process. In some cases this muscle is unusually well 

 developed. 



The sixth layer of muscles comprises the following : 



1. The complexus, or semispinalis capitis. 



2. The semispinalis dorsi in the dorsal region. 



3. The semispinalis colli in the cervical region. 



4. The multifidus spinae, found in all regions of the spine. 



5. The rotatores spinae, found in the thoracic region. 

 Describe the complexus muscle. 



This muscle (Figs. 283 and 284) is covered by the splenius, with its two 

 divisions, and the trapezius. In turn, this complexus covers the muscles that 

 make the boundaries of the suboccipital triangle, thereby forming the roof of this 

 important surgical area the suboccipital triangle. The occipital artery lies on 

 the muscle ; the suboccipital and great occipital nerves are under the muscle, 

 being parts of the contents of the suboccipital triangle. The origin of the muscle 

 is from : (i) The articular processes of the cervical vertebra? from the third to the 

 sixth ; the transverse processes of the seventh cervical and the six upper thoracic 

 vertebras. The insertion is into the occipital bone, between the middle and 

 inferior curved or nuchal lines. (Fig. 278.) Nerve-supply: (i) The suboccipital 

 or posterior primary division of the first cervical ; (2) the great occipital or 

 posterior primary division of the second cervical ; (3) the posterior primary 

 divisions of the third, fourth, and fifth cervical nerves. 



