830 ' THE VASCULAR SYSTEM. 
between the sternal and costal origins of the diaphragm, and terminate in the internal 
mammary veins. 
Musculo-phrenic veins.—The ven comites of the musculo-pbrenic artery com- 
mence in the abdomen, pass through the diaphragm with the artery, and terminate in 
the internal mammary veins. They receive as tributaries the anterior intercostal veins of 
the seventh, eighth, and ninth intercostal spaces, and small venules from the substance of 
the diaphragm. 
Vertebral Veins (vy. vertebrales)—These correspond only to the extra-cranial 
parts of the vertebral arteries. Each commences by the union of offsets from 
the intraspinal venous plexuses, and, issuing from the spinal canal, passes across 
the posterior arch of the atlas with the vertebral artery to the foramen in the 
transverse process of the atlas. It then descends through the foramina in the 
cervical transverse processes, and breaks up into a plexus of venous channels which 
surround the artery. At the lower part of the neck these channels unite to form 
a single trunk which issues from the foramen in the transverse process of the 
sixth cervical vertebra, and descends, in the interval between the longus colli and 
scalenus anticus muscles, to terminate in the upper and back part of the innominate 
vein, where it possesses a uni- or bi-cuspidate valve. 
Relations.—In the first part of its course the vein lies in the suboccipital triangle. 
The second, plexiform portion, is in the canal formed by the foramina in the transverse 
processes of the cervical vertebrae, and, with the artery which it surrounds, lies in front of 
the trunks of the cervical spinal nerves. The third part, in the root of the neck, is 
between the longus colli and scalenus anticus muscles, in front of the first part of the 
vertebral artery, ‘and behind the internal jugular vein. 
Tributaries.—In addition to the offsets from the intr: spinal venous plexuses by the 
union of which it is formed, each vertebral vein receives the following tributaries :—(a) 
Small vessels which issue from the muscles, hgaments, and bones of the deeper parts of the 
neck, and the lower and back part of the head. (4) Offsets from the intraspinal venous 
plexuses which pass out of the spinal canal by the intervertebral foramina.  (c) The 
anterior deep cervical or anterior vertebral vein, a vessel which is formed by the union of 
tributaries which issue from a venous plexus which lies in front of the bodies and 
on the roots of the transverse processes of the cervical vertebree. ‘This vessel accompanies 
the ascending cervical artery, and terminates in the lower part of the vertebral vein, 
immediately after the latter has issued from the foramen in the sixth cervical transverse 
process. (d) The posterior deep cervical (vy. cervicalis profunda) or posterior vertebral 
vein; this commences in the suboccipital triangle from a venous plexus with which 
the vertebral and occipital veins communicate. It descends behind the transverse 
processes of the cervical vertebree in company with the profunda cervicis artery, turns 
forwards at the root of the neck, between the transverse processes of the sixth and 
seventh cervical vertebrae or between the latter and the neck of the first rib, and opens 
into the vertebral vein. It receives blood from the muscles, ligaments, and bones of the 
back of the neck. (e) The posterior intercostal vein from the first intercostal space some- 
times opens into it. 
Occasionally the venous plexus round the vertebral artery ends below in two terminal 
trunks, anterior and posterior, instead of one. In these cases the second terminal vessel 
lies behind the lower part of the vertebral artery, passes through the foramen in the 
transverse process of the seventh cervical vertebra, and turns forwards on the outer side 
of the artery to join the anterior trunk, thus forming a common terminal vein which 
ends in the usual manner. ; 
Inferior Thyroid Veins (vy. thyreoidew inferiores)—Each inferior thyroid 
vein commences by the union of a series of tributaries which issue from the 
isthmus and the corresponding lateral lobe of the thyroid body. The two veins 
descend along the front of the trachea into the superior mediastinum, where the 
right inferior thyroid vein terminates in the junction of the two innominate veins, 
and the left in the upper part of the left innominate vein ; or the two veins unite 
to form a single trunk, which usually ends in the left innominate vein, but occa- 
sionally in the right. In their descent through the neck the inferior thyroid veins 
frequently anastomose together, and sometimes these anastomoses are so frequent 
and irregular that a venous plexus is formed in front of the lower cervical portion 
of the trachea. 
