THE THYROID AXIS. 



765 



spinal canal, where it anastomoses upon the surface of the spinal cord with the spinal branches 

 of the vertebral and of the intercostal arteries. 



In all its relations the deep cervical is comparable to a posterior branch of an intercostal 

 artery, and is to be regarded as the posterior branch of the seventh cervical segmental artery, 

 which is the subclavian. 



{d) The first intercostal artery passes outward and forward in the first intercostal space, 

 and resembles in its course and distribution an aortic intercostal (page 792). 



( c) The second intercostal artery arises at the bifurcation of the superior intercostal and 

 passes downward over the neck of the second rib to the second intercostal space, in which it 



Fig. 704. 



Deltoid 



Biceps, long head 



Anterior circumflex 



Biceps, short head 



Axillary artery 



Posterior 



circumflex 



Subscapular 



Dorsalis scapulae 



Long thoracic 

 Subscapularis 



Teres major 

 Latissimus dorsi 



Serratus magnus 



i "^ Common carotid 



Suprascapular 



Inferior thyroid 

 Thyroid axis 



Vertebral 

 Thyroid body 

 Common carotid 

 Trachea 



Subclavian artery 

 Clavicular facet 



of sternum 

 Int. mammary 



First rib 

 Superior thoracic 



Alar thoracic 



( Pectoralis minor. 

 cut 



Deep dissection exposing subclavian and axillary arteries and their branches. 



courses similarly to an aortic intercostal (page 792). It usually receives an anastomosing branch 

 from the third intercostal artery or else directly from the thoracic aorta, and may be replaced 

 by it. 



Variations. — The superior intercostal may arise from the vertebral artery and may termi- 

 nate in the first intercostal alone, the second arising from the third or from the thoracic aorta. 

 Anastomoses occur between the first and second intercostals and the arteria aberrans (page 

 792), when that vessel is present. 



4. The Thyroid Axis. — The thyroid axis Ttruncus thyreocervicaUs) (Fig. 704) 

 arises from the upper border of the subclavian, usually just medial to the medial 

 border of the scalenus anticus. It' ascends vertically upward for from 2—10 mm., 



