THE DISSECTION OF THE BACK 79 



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 interspinous ligaments. They should be laid 

 aside for the present. A description of the ligaments will be 

 found on p. 269. Whilst the specimen is fresh, however, the 

 dissector should test the high elasticity of the ligamenta flava by 

 stretching them. 



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. 



Arterise Spiuales. 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 vertebral column. In the cervical region they come from 

 the vertebral artery ; in the thoracic region, from the posterior branches 

 of the intercostal arteries ; in the lumbar region, from the dorsal branches 

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

 the bones, the periosteum, and the ligaments ; and their arrangement is 

 very much the same in each of the three regions. 



Each spinal artery may be looked upon as giving off three main twigs ; 

 one of them, termed the pre-laminar branch, is a very small twig which 

 ramifies upon the deep surface of the vertebral arches and ligamenta 

 flava. Another, the neural branch, can be followed to the dura mater, 

 which it pierces immediately above the point of exit of the corresponding 

 spinal nerve. It divides into two twigs, one of which passes along the 

 posterior and the other along the anterior root of the nerve to join the 

 plexus in the spinal pia mater. The third, the post-central branch, is 

 carried medially, anterior to the dura mater, towards the posterior surface 

 of the vertebral bodies ; it divides into an ascending and a descending twig 

 which anastomose with the corresponding twigs above and below, and in 

 that manner a continuous series of minute arterial arcades is formed, 

 from which branches pass medially to form a series of cross anastomoses 

 with the corresponding vessels of the opposite side. 



In the cervical region small branches from the ascending cervical artery 

 also find their way into the vertebral canal ; whilst in the sacral portion 

 of the canal the dissector will find branches from the lateral sacral arteries. 



Internal Vertebral Venous Plexus. The internal vertebral venous 

 plexus extends along the whole length of the vertebral canal, and consists 

 essentially of four subsidiary longitudinal plexuses, two anterior and two 

 posterior, which anastomose freely with each other. 



The posterior plexuses are united by many cross branches, which run 

 along the deep aspect of the vertebral arches and ligamenta flava. 

 Above, they communicate with the occipital sinus, whilst, all the way 

 down, they are connected with the posterior vertebral venoiis plexus by 

 wide channels which pierce the ligamenta flava. Laterally, they send 

 branches through the intervertebral foramina to join the posterior branches 

 of the intercostal and lumbar veins. 



The anterior plexuses cannot be dissected whilst the medulla spinalis 



