86o HUMAN ANATOMY. 



trapezius muscle near its origin from the superior nuchal line, and enters the suboccipital tri- 

 angle where it opens into the deep cervical vein. Occasionally, however, it either unites with 

 the posterior auricular vein or opens directly into the external jugular below the posterior 

 auricular. The mastoid emissary \ein (page S76) usually opens into one of its branches. 



2. The Vertebral Vein. — The vertebral vein (v. vertebralis) accompanies 

 the artery of the same name through all but the cranial portion of its course, and is 

 usually a single trunk, although frequently it is double or occasionally even plexiform 

 throughout more or less of its course. It arises in the suboccipital triangle from a, 

 plexus of small veins with which the occipital and deep cervical veins also communi- 

 cate, and passes downward through the foramina in the transverse processes of the six 

 (occasionally seven or five or even only four) upper cervical vertebrae. At its exit 

 from the foramen of the sixth vertebra it is continued obliquely forward and down- 

 ward behind the inferior thyroid artery and the internal jugular vein, and, passing 

 usually in front of, but occasionally behind, the subclavian artery, opens into the 

 innominate vein near its origin. 



The opening into the innominate is guarded by a pair of valves. Throughout 

 its course the vein is connected to the periosteum, lining each of the vertebrarterial 

 canals it traverses, by fibrous bands, and in its terminal portion it is adherent to the 

 deep cervical fascia, so that its walls do not collapse even when it is emptied of blood. 



Tributaries. — Like the vertebral artery, the vein receives tributaries from the deep mus- 

 cles of the neck and, at each intervertebral foramen which it passes, communicating branches 

 from the plexuses in the spinal canal on the one hand, and from the posterior spinal plexus and 

 the deep cervical vein on the other. In its terminal portion, after it has issued from the fora- 

 men in the transverse process of the sixth cervical vertebra, it receives the ascending cervical 

 vein, which arises in the plexus upon the anterior surfaces of the bodies of the upper cervical 

 vertebrae, and accompanies the ascending cervical artery down the neck. Very frequently it 

 also receives, shortly before its termination, the deep cervical vein. 



3. The Internal Mammary Vein. — The internal mammary vein (v. mamina- 

 ria interna) is formed by the vmion of the vense comites of the musculo-phrenic and 

 superior epigastric arteries, and throughout the greater part of its course is double, one 

 stem lying along the outer and the other along the inner side of the artery in its 

 course along the inner surface of the anterior thoracic w'all. Opposite the second 

 or third intercostal space the two stems unite, the single vein so formed lying to the 

 inner side of the artery and opening above into the innominate vein of the same 

 side. Numerous valves occur in the course of the vein. 



Tributaries. — The tributaries of the internal mammary veins correspond in general with 

 the branches of the internal mammary artery, with the exception of the superior phrenic, medi- 

 astinal, pericardial, and thymic branches, which usually open independently into the left innom- 

 inate vein. Its sternal branches form plexuses upon both surfaces of the sternum, and so form 

 communication with the vein of the opposite side, and the anterior intercostal branches unite 

 with the posterior intercostals ( page 896 ) . The perforating branches assist in returning the blood 

 from the pectoral muscles, those of the first and second intercostal spaces being larger than the 

 rest in the female, and serving to return a considerable portion of the blood from the mammary 

 gland. By means of the superior epigastric branches the internal mammary makes connection 

 with the subcutaneous veins of the abdomen, and, since these are also connected with the 

 epigastric and circumflex iliac branches of the iliac veins, an anastomosis is formed between the 

 superior and inferior caval systems o{ veins. 



4. The Inferior Thyroid Veins. — The inferior thyroid veins (vv. thyreoideae 

 inferiores) have their origin in a venous plexus (plexus thyreoideus irapar ) which covers 

 the anterior surface and sides of the trachea immediately below the isthmus of the 

 thyroid gland, the vessels which form the plexus issuing from the substance of the thy- 

 roid gland, or in some cases being downward prolongations of the branches of origin 

 of the superior thyroid veins. From the plexus two or sometimes three veins descend 

 the neck, following paths quite distinct from those of the inferior thyroid arteries, 

 and open below into the innominate veins, their orifices being guarded by valves. 

 When three veins are present, the odd one occupies a median position and is known 



