THE HEAD AND NECK 1141 



superior intercostal. The first three arise from the first part of the 

 artery towards its termination, and the last arises from the second 

 part on the right side, and from the first part on the left side. 



Vertebral Artery. — This is the first branch in order. On the right 

 side it arises from the upper and back part of the artery about 

 an inch from its origin, and on the left side from the vessel just after 

 it enters the root of the neck. It passes for a short distance upwards, 

 backwards, and slightly outwards, and disappears from view by 

 entering through the costo-transverse (vertebrarterial) foramen 

 in the transverse process of the sixth cervical vertebra as a rule. 

 It then ascends vertically through the successive costo-transverse 

 foramina above that of the sixth vertebra, and in passing from that 

 of the axis to that of the atlas it describes a bend outwards. Having 

 traversed the costo-transverse foramen of the atlas, it passes back- 

 wards and inwards, lying in the vertebrarterial groove on the upper 

 surface of the posterior arch of the atlas behind the superior articular 

 process. Thereafter it pierces the dura mater and arachnoid, and, 

 having reached the side of the spinal cord, it turns upwards in the 

 subarachnoid space, and enters the cranial cavity through the 

 foramen magnum. Being now related to the medulla oblongata, 

 it gradually inclines from its lateral to its ventral aspect, and at the 

 lower border of the pons Varolii it joins its feUow to form the basilar 

 artery. 



On account of its complicated course, the vertebral artery is 

 divided into four parts, namely, first or cer\'ical, second or costo- 

 transverse, third or suboccipital, and fourth or intracranial. 



The first or cervical part extends from the origin of the vessel to 

 the costo-transverse foramen in the transverse process of the sixth 

 cervical vertebra. It lies between the scalenus anticus and longus 

 colli muscles, having the internal jugular and vertebral veins in 

 front of it ; being crossed by the inferior thyroid artery ; and having 

 the sympathetic gangliated cord and the transverse process of the 

 seventh cervical vertebra behind it. The vessel of the left side 

 has the thoracic duct as an additional anterior relation. 



The first part gives off no branches. 



The second or costo-transverse part traverses the costo-transverse 

 foramina of the cervical vertebrae from, as a rule, the sixth upwards. 

 It is surrounded by the vertebral venous plexus, and by the vertebral 

 plexus of the sympathetic, and it lies in front of the cervical spinal 

 nerves as these emerge from the intervertebral foramina. 



Branches. — ^These are as follows : lateral spinal and muscular. 

 The lateral spinal branches enter the spinal canal through the 

 inter\'ertebral foramina. The muscular branches supply the deep 

 muscles of the neck, and anastomose with the deep cervical, ascend- 

 ing cervical, and occipital arteries. 



For the third or suboccipital, and the fourth or intracranial, parts 

 of the vertebral artery, see Index. 



Varieties. — (i) The left vertebral artery occasionally arises from the arch 

 of the aorta between the origins of the left common carotid and left subclavian 



