THE DISSECTION OF THE BACK 193 



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. By pulling upon the dura mater 

 the whole specimen may now be lifted from the vertebral canal and trans- 

 ferred to the water-bath. The dura mater should then be slit down 

 the median plane, and the edges of the incision must be turned aside. 

 By fixing the dura mater with pins to the cork at the bottom of the tray, 

 the further dissection can be conducted with great advantage. 



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. 



A large number of small arteries are supplied to -the 

 medulla spinalis. These are the anterior and posterior spinal 

 arteries which spring from the vertebral 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 

 these arterial twigs, five longitudinal trunks are formed upon 

 the surface of the medulla spinalis. One of these lies in 

 the median plane anteriorly, and may be termed the antero- 

 median artery. The other four are placed in relation to the 

 sulci along which the posterior nerve-roots enter the medulla 

 spinalis. One runs downwards anterior to the line of entrance 

 of these roots, and the other posterior to it on each side of 

 the medulla spinalis. These slender arterial trunks may there- 

 fore be termed the postero-lateral longitudinal vessels. 



The antero-median vessel is formed above by the union of the two 

 anterior spinal branches of the vertebral arteries. One of these is larger 

 than the other, and takes a much greater share in the formation of the 

 median trunk. Below the level of the fifth pair of cervical nerves the 

 continuity of the median vessel depends upon the reinforcements which 

 it obtains from the lateral spinal vessels. The number of lateral spinal 

 arteries which join the median vessel is very variable. The majority 

 of these arteries end on the nerve-roots ; only five to ten reach the median 

 vessel. The antero-median artery runs downwards, under cover of the 

 linea splendens of the pia mater. Its calibre is uniform throughout, 

 and where the medulla spinalis ends it proceeds onwards for some distance 

 upon the filum terminale. 



^\it postero-lateral arteries on each side of the medulla spinalis are formed 

 in the upper part of the cervical region by the bifurcation of the corre- 

 sponding posterior spinal branch of the vertebral artery. Lower down 

 their continuity is maintained by twigs which reach them on the posterior 

 roots of the spinal nerves from the lateral spinal arteries. It may be 

 VOL. II 13 



