86 ANTHROPOLOGY. 



its origin to the scalenus, is nearly vertical, and lias anterior to it tlie sterno- 

 mastoid, hyoid and thyroid muscles, the clavicle and first rib, the left vena 

 innominata, left carotid artery, the left lung and pleura, &c. 



The subclavian artery, in the middle stage of its course, on each side of 

 the neck is covered by the sterno-mastoid and anterior scalenus muscle, and 

 lies on the pleura and middle scalenus : the subclavian vein is inferior and 

 anterior. 



In the third stage it inclines downwards and outwards, and is covered 

 only by the skin, platysma myoides, fascia, and cellular tissue ; it rests on 

 the middle scalenus and the first rib. The vein is inferior and anterior, 

 concealed by the clavicle. The subclavian arteries in their course send off 

 the following branches : vertebral, thyroid axis, internal mammary, superior 

 intercostal, and deep cervical. 



The vertebral artery arises from the upper and back part of the subcla- 

 vian, and, ascending, enters the foramen in the transverse process of the 

 fifth or sixth cervical vertebra, continuing through the corresponding cervi- 

 cal foramina as far as the second vertebra. Bending backwards in an 

 exceedingly tortuous manner, it passes through the foramen in the trans- 

 verse process of the atlas, and alternately piercing the dura mater, advances 

 within the cranium through the foramen magnum, and unites with the 

 opposite at the lower edge of the pons varolii to form the basilar artery. In 

 this course it gives off small branches to the spinal nerves, and to the inter- 

 vertebral muscles ; at the foramen magnum it gives off, first and second, the 

 posterior and inferior sjnnal arteries, which descend all along the spinal 

 cord ; third, the inferior cerebral artery, which runs tortuously around the 

 medulla oblongata, sending off numerous branches to the inferior surface of 

 the cerebellum. 



The basilar artery is formed by the confluence of the two vertebral ; it 

 ascends in the median groove on the pons varolii, and at the upper edge of 

 that body divides into four branches, two for each side : first, the superior 

 cerebellar artery, passing backwards to the upper surface of each hemisphere 

 of the cerebellum, in which it spreads its branches ; second, the posterior 

 cerebral artery, which receives the posterior branch of the internal carotid, 

 bends backwards and outwards, and spreads its ramifications on the posterior 

 lobe of the cerebral hemisphere. The communication between these poste- 

 rior cerebral arteries and the posterior branches of the internal carotids, 

 completes that remarkable inosculation known as the circle of Willis; this 

 is formed anteriorly by the two cerebral arteries, with their cross uniting 

 branch ; laterally by each internal carotid, and its posterior communicating 

 branch ; and posteriorly by the trunk of the basilar, and the roots of the 

 posterior cerebral arteries. 



The thyroid axis arises from the upper part of the subclavian close to the 

 scalenus muscle and phrenic nerve ; it immediately divides into the four 

 following branches : 1. The inferior thyroid, distributed to the thyroid gland, 

 sending branches also to the trachea, oesophagus. &c. ; 2. The ascending 

 cervical, distributed to the anterior scalenus, longus colli, and rectus capitis 

 anticus major muscles ; 3. Supra-scapular, which supplies the supra- spinatus 

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