THE BRACHIAL OB AXILLARY ARTERIES. 563 



Ml canal with the retrograde branch of the occipital artery, at the atlo-axoid 

 articulation, underneath the great oblique muscle of the head. 



In its track, it detaches at each intervertebral space numerous branches, 

 which may be divided into inferior, superior, external, and internal. The 

 first chiefly pass to the scalenus, longus colli, and the great anterior rectus 

 muscle of the head. The second, which are incomparably larger and more 

 numerous than all the others, are destined to the two complex muscles, the 

 transverse-spinous (semispinalis) muscles of the neck, and to the ilio-spinal 

 muscle ; they anastomose with the divisions of the superior cervical and 

 occipito-muscular arteries. The external branches, are very small, and 

 pass to the intertransverse muscles. The iuternal branches enter the inter- 

 vertebral foramina to join the middle spinal artery. 



4. Internal Thoracic, or Internal Mammary Artery. (Fig.' 282, 9.) 



The internal thoracic artery emerges from the brachial trunk at the 

 first rib, and immediately descends along the inner face of that bone to the 

 sternum, remaining covered by the pleura. It then bends backwards, passes 

 under the triangular muscle and above the sternal cartilages, which it 

 crosses near the chondro-sternal articulation, and reaches the base of the 

 xiphoid appendix, where it ends in two branches : one abdominal, the other 

 thoracic, and which have been named the anterior abdominal and asternal 

 arteries. 



In its course, the internal thoracic artery sends off collateral branches 

 which may be distinguished into superior, inferior, and external. The 

 superior are always very slender, and proceed to the pericardium and 

 mediastinum. The inferior (Fig. 282, 11) are very large, and traverse the 

 intercostal spaces to enter the pectoral muscles, where they meet the ramifi- 

 cations of the external thoracic artery. The external branches (Fig. 282, 10) 

 follow the intercostal spaces ; each generally divides into two branches, 

 which finally anastomose by inosculation with the terminal divisions of the 

 first seven intercostal arteries. 



Terminal branches of the internal thoracic artery. — 1. Anterior ab- 

 dominal artery. — This vessel separates from the asternal artery at an acute 

 angle, and passes directly backward to escape from the chest by coursing 

 beneath the xiphoid appendix ; it then places itself on the superior face of 

 the rectus muscle of the abdomen, which it enters, after detaching lateral 

 branches to the abdominal walls, and anastomoses by its terminal ramifica- 

 tions with the posterior abdominal artery. 



2. Asternal artery. — This vessel glides within the cartilaginous circle 

 formed by the false ribs, in crossing the digitations of the transverse muscle 

 of the abdomen, and terminates at the thirteenth intercostal space, in which 

 it ascends to anastomose with the corresponding intercostal artery. It 

 supplies in its track : intercostal branches, which comport themselves like 

 the analogous branches of the internal thoracic artery ; fine diaphragmatic 

 twigs ; and abdominal divisions, which particularly ramify in the tranverse 

 muscle. 



5 External or Inferior Thoracic, or External Mammary Artery. 



(Fig. 282, 12.) 



Principally destined to the deep pectoral muscles, this artery commences 



at an acute angle in front of, but close to, the preceding, turns the anterior 



border of the first rib, and then passes back against the internal face of the 



great pectoral and sterno-prescapular muscles, in which are expended its 



