742 THE VASCULAR SYSTEMS 



in large, tortuous channels in the substance of the bones, similar in every respect 

 to those found in the diploe of the cranial bones. These canals lie parallel to the 

 upper and lower surfaces of the bones. They commence by small openings on the 

 front and sides of the bodies of the vertebrae, through which communicating 

 branches from the veins external to the bone pass into its substance, and converge 

 to the principal canal, which is sometimes double toward its posterior part. They 

 open into the corresponding transverse branch uniting the anterior longitudinal 

 veins. They become greatly enlarged in advanced age. 



4. The veins of the spinal cord (vv. spinales] emerge from the cord substance 

 and form a minute, tortuous, venous plexus which covers the entire surface of the 

 cord, being situated between the pia and arachnoid. In this plexus there are 

 (1) two median longitudinal veins, one in front of the ventral fissure, and the other 

 behind the dorsal groove of the cord; and (2) four lateral longitudinal veins which 

 run behind the nerve roots. These vessels are largest in the lumbar region. 

 Near the base of the skull they unite, and form two or three small trunks, which 

 communicate with the vertebral veins, and terminate in the inferior cerebellar 

 veins or in the inferior petrosal sinuses. Each of the spinal nerves is accompanied, 

 by a radicular efferent vessel as far as the intervertebral foramina, where it joins 

 the other veins from the vertebral canal. 



There are no valves in the spinal veins. 



VEINS OF THE LOWER EXTREMITY, ABDOMEN, AND PELVIS 



(Figs. 518, 519). 



The veins of the lower extremity are subdivided, like those of the upper, into 

 two sets, superficial and deep; the superficial veins are situated beneath the integu- 

 ment, between the two layers of superficial fascia, the deep veins accompanying 

 the arteries, and forming the venae comites of those vessels. Both sets of veins 

 are provided with valves, which are more numerous in the deep than in the super- 

 ficial set. These valves are also more numerous in the lower than in the upper 

 limb. 



The Superficial Veins of the Lower Extremity. 



The Superficial Veins of the Foot. In the sole of the foot there is a sub- 

 cutaneous venous plexus (rete venosum plantare cutaneum), from which some 

 branches go to the deep veins, but most of the branches pass around the margins 

 to the dorsum of the foot. There is a transverse venous arch at the root of the 

 toes which receives plantar vessels from the toes and sends branches between the 

 toes (vv. intercapitulares] to the venous arch of the dorsum. On the dorsum of 

 each toe the veins gather into two vessels, known as the dorsal digital veins (vv. 

 digitales pedis dorsalis}. The dorsal digital veins from the opposed margins of 

 two toes unite to form a dorsal interdigital vein. There are four dorsal inter- 

 digital veins (vv. digitales communes pedis], and they pass into the venous arch 

 of the dorsum. The dorsal digital vein, from the inner surface of the great toe, 

 passes directly into the internal saphenous vein, and the dorsal digital vein, from 

 the outer surface of the little toe, passes directly into the external saphenous vein. 



The venous arch of the dorsum of the foot (arcus venosus dorsalis pedis [cutaneus]), 

 is situated in the superficial structures over the anterior extremities of the meta- 

 tarsal bones. It has its convexity directed forward, and receives digital tributaries 

 from the upper surface of the toes; at its concavity it is joined by numerous small 

 veins which form a plexus on the dorsum of the foot (rete venosum dorsale pedis 

 cutaneum). The arch terminates internally in the long saphenous. externally 

 in the short saphenous vein. 



