THE DISSECTION OF THE BACK 181 



tension 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 j then it pierces the dura mater and enters the 

 vertebral canal. 



Small branches proceed from the vertebral artery in this 

 situation 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 processes upon both sides. 

 The multifidus must be completely removed from the dorsum of the 

 sacrum. At the same time the posterior branches of the nerves must 

 be retained, so that their continuity with the various spinal nerve-trunks 

 may be afterwards established. The posterior wall of the vertebral canal 

 should now be removed in one piece by sawing through the laminae on 

 either side, and dividing the ligamenta flava, from the third cervical 

 vertebra down to the lower opening of the canal on the back of the sacrum. 



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) in cutting through the cervical laminae 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 block must be placed 

 under the abdomen of the subject, whilst the blocks supporting the chest 

 and pelvis are removed. It will probably be necessary at this point to 

 have recourse to the hammer and chisel. 



The laminae and spinous processes which are thus removed are connected 

 with each other by the ligamenta flava and the supraspinous and inter- 

 spinous ligaments. They should be laid aside for the present. A de- 

 scription of these ligaments will be found on p. 358. When the specimen 

 is fresh, however, the dissector should test the high elasticity of the 

 ligamenta flava by stretching the specimen. 



Between the dura mater and the walls of the canal, the dissector will 

 notice a quantity of loose areolar tissue and soft fat. The latter is 

 especially plentiful in the" sacral region, where it somewhat resembles the 

 marrow in the medullary cavity of a long bone. Great numbers of large 

 veins and minute arteries ramify in this areolo-fatty material. 



Arteriae Spinales. In a well-injected subject a minute 

 spinal artery will be seen entering the vertebral canal through 

 each intervertebral foramen. These arteries are derived from 

 different sources in the different regions of the spine. In the 

 cervical region they come from the vertebral artery, and in 

 the thoracic region from the posterior branches of the inter- 

 costal arteries ; in the lumbar region from the dorsal branches 

 of the lumbar arteries. They supply the spinal medulla and its 



n125 



