90 HEAD AND NECK 



The nerve-trunk is exceedingly short in most cases; 

 indeed, it divides almost immediately into its anterior and 

 posterior rami. In the cases of the sacral and coccygeal nerves, 

 the subdivision takes place in the sacral canal, and the 

 spinal nerve-trunks of those nerves are distinctly longer than 

 the trunks of the nerves which occupy a higher level. 



The distribution of the posterior rami has already been 

 examined (p. 69). 



Dissection. At this stage the dissector may adopt one of two 

 methods in the further treatment of the medulla spinalis and 

 the nerves which spring from it. If the medulla spinalis is fresh 

 and in such a condition that it may be successfully hardened, it 

 is best to transfer it at once to the preservative fluid. If, on the 

 other hand, it is soft and not fit for proper preservation, it should 

 be removed with all its membranes and nerve-roots, and placed 

 in a cork-lined tray filled with water. There is no method by 

 which the arachnoid, the pia mater, the ligamenta denticulata, 

 and the nerve-roots can be so well studied as this. 



To remove the medulla spinalis, the dissector should divide 

 the spinal nerves as they lie in the intervertebral foramina, and 

 in such a manner that as long a piece as possible of each nerve 

 remains attached to the dura mater and the spinal medulla. 

 Wherever it is possible the ganglia should be taken with the 

 nerves. The same rule applies to the sacral nerves also. The 

 medulla spinalis and its membranes should then be cut across 

 at the highest limit of the vertebral dissection. Pull upon the 

 dura mater in order to lift the whole specimen from the vertebral 

 canal, and then transfer it to the water-bath. Slit up the dura 

 mater along the median plane anteriorly, and turn aside the edges 

 of the incision. By fixing the dura mater with pins to the cork 

 at the bottom of the tray, the dissector can conduct the further 

 dissection with great advantage, and can display in turn the 

 arachnoid, and the pia mater with the ligamenta denticulata. 



Arteries of the Medulla Spinalis (O.T. Spinal Cord). It 

 is only when the arterial injection is particularly good that 

 the spinal arteries can be made out satisfactorily. 



Numerous small arteries are supplied to the medulla 

 spinalis. They are the anterior and posterior spinal arteries, 

 which spring from the vertebral artery in the cranium, and a 

 series of lateral spinal arteries, which reach the side of the 

 medulla spinalis and are derived from different sources in each 

 region. In the neck they come from the vertebral, ascending 

 cervical, and deep cervical arteries ; and in the thoracic and 

 lumbar regions, from the posterior branches of the intercostal 

 and lumbar arteries. By the anastomoses of the various 

 arterial twigs, five longitudinal trunks are formed upon the 

 surface of the medulla spinalis. One lies in the median 



