THE DISSECTION OF THE BACK 91 



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 those 

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

 medulla spinalis. The posterior vessels may, therefore, 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 them 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 them 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. 



The 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 

 regarded as a rule, that where a lateral spinal artery gives a branch to 

 one of the postero-lateral arterial trunks, it does not furnish another 

 to the antero-median arterial trunk. Nevertheless, the different lateral 

 spinal arteries are in connection, directly or indirectly, with the longitudinal 

 trunks on the anterior and posterior aspects of the medulla spinalis. The 

 postero-lateral vessels end at the lower extremity of the medulla spinalis. 



F r om the five main arterial channels which thus extend along the spinal 

 medulla spring numerous anastomosing twigs which ramify in the pia 

 mater. 



Veins of the Medulla Spinalis. The veins of the spinal 

 medulla are small and numerous, and their disposition cannot 

 be said to correspond with that of the arteries. They are 

 very tortuous, and form a plexus with elongated meshes. 

 Six more or less perfect longitudinal venous trunks may be 

 noticed on the surface of the medulla spinalis in connection 

 with the venous plexus; two of them are median, and are 

 placed respectively on the anterior and posterior aspects. 

 The anterior trunk runs upwards under cover of the antero- 

 median spinal artery. The other four are lateral, and are 

 situated two on each side, in relation, respectively, to the 

 anterior and posterior nerve-roots. 



Upon each side, the veins of the medulla spinalis effect 



