DISSECTION OF THE HEAD AND NECK. 173 



THE REGION OF THE POLL. 



Directions. — It will be convenient at this stage to dissect a group of 

 muscles (with their nerves and vessels) placed above the occipito-atlantal 

 and atlanto-axial joints (Fig. 20, page 160). The cervico-auricular 

 muscles of one side having been cleared away, the mastoid tendon of the 

 mastoido-humeralis (page 156) will present itself. Beneath that, again, 

 is the mastoid tendon common to the splenitis and trachelo-mastoideus 

 (page 157); and still deeper, there is the occipital insertion of the 

 complexus. Each of these, having been identified, may be cut away ; 

 and the following muscles are to be isolated, the nerves and vessels 

 being thereafter dissected on the other side. 



The Obliquus Capitis Inferior. This muscle, the most powerful of 

 the group, covers the atlanto-axial joint on each side. It arises horn the 

 superior spine of the dentata ; and it is inserted into the wing of the 

 atlas on its upper aspect. 



Action. — To rotate the atlas (and head) around the odontoid process 

 of the axis. 



The Obliquus Capitis Superior. This muscle covers the occipito- 

 atlantal joint on each side. It arises from the free edge of the wing of 

 the atlas ; and it becomes inserted into the mastoid crest and styloid 

 process of the occipital bone. 



Action. — To extend the head on the atlas. 



The Rectus Capitis Posticus Major. This muscle is composed of 

 two parallel portions which, although not distinctly separated from one 

 another, were described as distinct muscles by Percivall. It arises from 

 the spinous process of the axis. Its most superficial fibres (complexus 

 minor of Percivall) join the occipital insertion of the complexus ; while 

 its deeper portion (rectus capitis posticus major of Percivall) is inserted 

 into the back of the occipital bone, beneath the insertion of the com- 

 plexus. 



Action. — The same as the preceding muscle. 



The Rectus Capitis Posticus Minor. This is the smallest muscle of 

 the group. It lies beneath the preceding, and covers the occipito-atlantal 

 joint. It arises from the upper aspect of the ring of the atlas ; and it is 

 inserted into the back of the occipital bone, beneath the last described 

 muscle. 



Action. — The same as the two preceding muscles. 



The 1st Cervical Nerve (Suboccipital of man) issues from the spinal 

 canal by the antero-internal foramen of the atlas. At its point of 

 emergence it resolves itself into superior and inferior primary branches. 

 The latter is immediately directed down through the antero-external 

 foramen, and will be followed at a later stage. The superior primary 

 branch appears between the obliquus capitis superior aud the rectus 



