78 HEAD AND NECK 



the atlas and the vertebral artery, and at once breaks up 

 into branches which go to supply five muscles viz., the two 

 posterior recti, the two oblique muscles, and the semispinalis 

 capitis. In addition to the muscular twigs it gives a com- 

 municating, and sometimes a cutaneous filament. 



The communicating branch generally proceeds from the 

 nerve to the obliquus capitis inferior, and joins the greater 

 occipital nerve. The cutaneous branch, when present, accom- 

 panies the occipital artery to the integument over the occiput. 



Arteria Vertebralis. Only the third portion of the 

 vertebral artery lies in the sub-occipital triangle. It emerges 

 from the foramen in the transverse process of the atlas, and 

 runs backwards and medially in the groove upon the posterior 

 arch of that bone. As it passes medially it lies immediately 

 posterior to the lateral mass of the atlas and above the sub- 

 occipital nerve. It leaves the space by passing anterior to 

 the thickened lateral extension of the posterior atlanto- 

 occipital membrane, which runs from the posterior arch of 

 the atlas to the posterior lip of its articular process and is 

 called the oblique ligament of the atlas ; then the artery 

 pierces the dura mater and enters the spinal canal (Fig. 37). 



Small branches proceed from the vertebral artery, as it lies 

 in the sub-occipital space, to supply the parts in its immediate 

 neighbourhood, and to anastomose with the descending 

 branch of the occipital artery and the arteria profunda 

 cervicis. 



Dissection to open the Vertebral Canal. The first step 

 consists in thoroughly cleaning the laminae and spinous pro- 

 cesses upon both sides. The multifidus must be completely 

 removed from the dorsum of the sacrum. At the same time 

 the posterior rami of the nerves must be retained, so that their 

 continuity with the various spinal nerve-trunks may be after- 

 wards established. The dissector should then remove the 

 posterior wall of the vertebral canal in one piece by sawing 

 through the laminae on each side, and dividing the ligamenta 

 flava, from the third cervical vertebra down to the lower opening 

 of the sacral canal. 



In making this dissection the student must attend to the 

 following points : (i) the cut should be directed through the 

 laminae close to the medial sides of the articular processes ; 

 (2) the saw must be used in an oblique plane, so that the cut 

 through the laminae slants slightly medialwards ; (3) as the 

 cervical laminae are cut through, the head and neck should hang 

 over the end of the table, and be flexed as much as possible, whilst 

 the saw is worked from below upwards ; (4) in the case of the 

 lumbar region, where, indeed, most difficulty will be met, a high 



