THE VEINS OF THE UPPER EXTREMITY. 843 
and it gives off efferent vessels which pass through the intervertebral foramina to 
terminate, according to the region in which they are placed, in the vertebral, intercostal, 
lumbar, or lateral sacral veins. 
Superiorly the anterior longitudinal spinal veins give off large offsets, above the arch 
of the atlas, which form the commencement of the vertebral veins ; through the foramen 
magnum they communicate with the basilar and with the occipital sinuses. 
The posterior longitudinal spinal veins are placed, one on each side, between the 
dura mater anteriorly and the laminz and ligamenta subflava posteriorly. They are 
plexiform vessels w hich extend along the whole length of the spinal canal. They receive 
tributaries from the laminae, ligaments, and spinal ‘membranes, and from a post-vertebral 
plexus of veins which lies betw een the laminze of the vertebrze and the deep muscles of 
the back ; they anastomose with each other by transverse channels which pass across the 
laminze, with the posterior spinal plexus by vessels which pierce the ligamenta subflava, 
and with the anterior longitudinal veins round the margins of the canal. Their efferent 
vessels unite with those of the anterior longitudinal veins, and terminate with them. 
By means of the longitudinal spinal veins and the anastomoses between them, a 
venous ring is formed within the spinal canal opposite each vertebra. Commencing in 
front, opposite the body of the vertebra, where it receives the vena basis vertebra, it passes 
outwards to the anterior longitudinal spinal vein, turns backwards along the inner side 
of the pedicle and the inner surface of the lamina to the posterior longitudinal vein, and 
is completed by the anastomosis between the posterior longitudinal veins. This ring 
communicates through the ligamenta subflava with the posterior spinal plexus, and 
through the intervertebral foramina with the vertebral, with the dorsal tributaries of 
the intercostal or lumbar veins, or with the lateral sacral veins, according to the region 
in which it lies. 
Superiorly the posterior longitudinal spinal veins communicate with the occipital 
sinuses, and as these also communicate with the anterior spinal veins, and the latter 
with the basilar sinus, a venous ring is completed round the foramen magnum. 
Veins of the Spinal Cord.—The veins of the spinal cord issue from the 
substance of the cord, and terminate in a plexus in the pia mater. In this plexus 
there are six longitudinal channels—one antero-median, along the anterior 
fissure, two antero-lateral, immediately behind the anterior nerve roots, two 
postero-lateral, immediately behind the posterior nerve roots, and one postero- 
median, over the postero-septum. Radicular efferent vessels issue from the plexus, 
and pass along the nerve roots to communicate with the efferent vessels from the 
anterior and posterior longitudinal spinal veims, and to terminate in them. 
The veins of the spinal cord vary very much in size, but they are largest on the 
lower and on the posterior portions of the cord. 
The postero-median and antero-median veins are continued above into the 
corresponding veins of the medulla oblongata. 
The antero-lateral and postero-lateral veins pour their blood partly into the 
median veins and partly into the radicular veins; indeed, the greater part of the 
blood from the spinal cord is returned by the latter veins. 
THE VEINS OF THE UPPER EXTREMITY. 
The veins of each upper extremity are divisible into two sets—viz. superficial and 
deep. Both sets open eventually into a common terminal trunk which is known 
as the axillary vein. This vein is therefore the chief efferent stem of the upper 
extremity. It is continued as the subclavian vein to the innominate vein, and its 
blood, together with that of the corresponding side of the head and neck, reaches 
the superior vena cava. 
THE DEEP VEINS OF THE UPPER EXTREMITY. 
The deep veins, with the exception of the axillary vein, are arranged in pairs 
which accompany the different arteries and are similarly named. So far as these 
veins (ven comites) are concerned it will be sufficient to state that they are pro- 
vided with several valves, that they are situated one on either side of the artery 
with which they are associated, and that they are usually united together by 
