THE BACK. 409 



mastoid. The complexus will be seen arising from : (i) The back of the artic- 

 ular processes of the cervical vertebrae from the third to the sixth ; (2) the back 

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

 vertebrae ; (3) generally also by an inner head from the spine of the seventh cer- 

 vical vertebra. It is inserted into the occipital bone, between the superior and 

 inferior nuchal lines. (Fig. 283.) 



Carefully detach the insertion of this muscle and you will expose : (i) The 

 suboccipital triangle, containing the vertebral artery and the suboccipital nerve ; 

 (2) on the under part of the muscle you will find the suboccipital nerve and the 

 great occipital nerve ; (3) the recti and oblique muscles ; (4) the semispinalis colli, 

 coming to a large abrupt insertion into the spine of the axis. 



The suboccipital triangle has (Fig. 285) : 



A roof formed by the complexus muscle. 



A floor formed by the arch of the atlas and atlo-occipital ligament. 



An inner border formed by rectus capitis posticus major. 



A lower border formed by inferior oblique muscle. 



An upper border formed by superior oblique muscle. 



The contents are the vertebral artery and suboccipital nerve. 



It is traversed by the great occipital nerve. 



The suboccipital muscles (Fig. 285) : 



1. The obliquns capitis inferior, which extends from the spine of the axis to 

 the transverse process of the atlas. 



2. The obliquns capitis superior, which extends from the transverse process of 

 the atlas to the inferior nuchal line of the occipital bone. 



3. The rectus capitis posticus major, which extends from the spine of the 

 axis to inferior nuchal line, middle one-third. 



4. The rectus capitis posticus minor extends from the tubercle of the atlas to 

 the inner one-third of the inferior nuchal line. 



5. The rectus capitis later alls extends from the lateral mass of the atlas to the 

 jugular process of the occipital bone. 



Now remove from its origin (i) the complexus, and as you turn the body of 

 the muscle from the semispinalis colli, you will see between these two muscles 

 some vessels the anastomosis between the arteria profunda cervicalis, a branch 

 of the first or superior intercostal, and the arteria princeps cervicis, a branch of 

 the occipital. (Fig. 40.) You will find a large mass of veins here, too. In 

 cases of ligation of the subclavian or common carotid artery, this is the principal 

 channel by which the blood passes to form a collateral circulation. (2) Remove 

 any remaining part of the spinalis dorsi or longissimus dorsi and make your 

 work look like figure 285. 



You have remaining the other members of the sixth and seventh groups : 

 (i) The semispinalis dorsi ; (2) the semispinalis colli; (3) the multifidus spinae. 

 These were explained under the head of " Analysis of the Simple Movements of 

 the Spinal Column," to which you are referred on page 395. Remove the three 

 muscles just given, and dissect (4) the rotatores, (5) the intertransversales, (6) the 

 interspinales, according to their previous description, just referred to. 



i . Give the cutaneous nerve-supply of the back. 



The scalp is supplied by (i) the great occipital ; (2) the suboccipital ; (3) the 

 third occipital. These are all spinal nerves. The remainder is supplied by the 

 posterior divisions of the spinal nerves. Next the spine you will see some small 

 nerves coming through to the skin. (Fig. 279.) These are cutaneous twigs 

 from the internal branches of the posterior divisions of the spinal nerves. A 

 little distance from the spine you will see nerves coming through the trapezius 

 and latissimus dorsi. These are the external cutaneous branches of the posterior 

 divisions of the spinal nerves. 

 27 



