1142 A MANUAL OF ANATOMY 



arteries, (2) The vessel may pass the costo-transverse foramen of the sixth 

 cervical vertebra, and may enter that of the fifth, or even that of the fourth. 

 (3) The vessel has been found in rare cases to enter the costo-transverse 

 foramen of the seventh cervical vertebra. 



Development. — The vertebral artery is developed from (i) the seventh cer- 

 vical segmental artery, and (2) the longitudinal anastomotic chain which 

 connects the seven cervical segmental arteries. 



Internal Mammary Artery. — ^This vessel is so named in contra- 

 distinction to the external mammary, or long thoracic, which is a 

 branch of the second part of the axillary artery. It arises from 

 the lower aspect of the first part of the subclavian artery very nearly 

 opposite the thyroid axis. Its course is downwards, forwards, and 

 slightly inwards, and it disappears behind the sternal end of the 

 clavicle and first costal cartilage. For its subsequent course and 

 relations, see p. 931. 



Relations of Cervical Part — Anterior. — ^The clavicular part of the 

 sterno-cleido-mastoid, and the internal jugular and subclavian veins. 

 The phrenic nerve crosses the vessel superficially from without 

 inwards. Posterior. — ^The pleura. 



No branches arise from this part of the vessel. 



Thyroid Axis. — ^This is a short trunk which springs from the front 

 of the first part of the subclavian artery close to the inner border 

 of the scalenus anticus muscle. It almost immediately divides into 

 three diverging branches, namely, inferior thyroid, transverse 

 cervical, and suprascapular. 



The inferior thyroid artery passes upwards and inwards in a 

 tortuous manner in front of the vertebral artery, and b'ehind the 

 carotid sheath and sympathetic nerve, the middle cervical ganglion 

 of which latter rests upon it. Having reached the lower part of the 

 lateral lobe of the thyroid body, the artery breaks up into its terminal 

 branches, which enter the lobe on its deep aspect, and ramify in it, 

 anastomosing with the superior thyroid, and with its fellow of the 

 opposite side in the isthmus. As the vessel ascends it is intimately 

 related to the recurrent laryngeal nerve, which usually lies behind it. 



Branches. — ^These are as follows : muscular, ascending cervical, 

 inferior laryngeal, tracheal, and oesophageal. 



The muscular branches supply the scalenus anticus, longus colli, 

 depressor muscles of the hyoid bone, and inferior constrictor of the 

 pharynx. The ascending cervical artery arises from the inferior 

 thyroid as it passes inwards behind the carotid sheath. It ascends 

 in front of the transverse processes of the cervical vertebrae, lying 

 between the scalenus anticus and rectus capitis anticus major, and 

 having the phrenic nerve on its outer side. It anastomoses with 

 branches of the vertebral, occipital, and ascending pharyngeal 

 arteries. It may furnish some spinal branches which enter the 

 spinal canal through the intervertebral foramina, to be distributed 

 in a manner similar to the other spinal arteries. The inferior 

 laryngeal artery accompanies the recurrent laryngeal nerve to the 

 larynx. The tracheal and oesophageal branches are distributed as 

 their names suggest. 



