86 ANTHROPOLOGY. 
its origin to the scalenus, is nearly vertical, and has anterior to it the sterno- 
mastoid, hyoid and thyroid muscles, the clavicle and first rib, the left vena 
imnominata, 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 andl 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 8 is comaell 
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 eat eal 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 basilarartery. 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 spinal arteries, which descend all along the spinal 
- cord; third, the mferior 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 
sealenus muscle and phrenic nerve; it immediately divides into the four 
following branches: 1. The m/erior thyroid, distributed to the thyroid gland, 
sending branches also to the trachea, cesophagus, &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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