DISSECTION OF THE BACK AND THORAX. 115 



The Left Axillary Artery is smaller than the other division of 

 the anterior aorta, and placed at a higher level. It passes forwards in 

 the anterior mediastinum, describing a curve which has its convexity 

 directed upwards and forwards. It leaves the chest by passing to the 

 inner side of the 1st rib, and turns round the anterior border of the 

 bone, where it leaves a smooth impression below the lowest fibres of the 

 scalenus. From this point it is directed downwards and backwards 

 across the inner aspect of the shoulder, beyond which it is continued as 

 the brachial or humeral artery. The vessel has thus a part within the 

 thorax, and another in the axilla ; but only the former presents itself 

 now. In the human subject the artery passes beneath the clavicle and 

 is termed the Subclavian. The arch formed by the thoracic part of the 

 left axillary rests to its right on the trachea, and touches at its highest 

 point the oesophagus. Beneath the arch the vagus and phrenic nerves 

 and one of the left cardiac nerves pass backwards. The common trunk 

 - of the left dorsal, superior cervical, and vertebral veins crosses it on the 

 % left, in passing down to the anterior vena cava. The collateral branches of 

 the artery arising within the chest are four in number, and three of them, 

 viz., the dorsal, superior cervical, and vertebral arteries, arise from the 

 summit of the arch ; the other, the internal thoracic or mammary artery, 

 takes origin from its lower aspect, at the hinder edge of the 1st rib. 



The Dorsal Artery passes upwards and forwards across the oesophagus 

 and longus colli muscle, and disappears at the upper end of the 2nd 

 intercostal space. At the outer edge of the longus colli it gives off the 

 subcostal artery, a vessel which furnishes the 2nd, 3rd, and 4th inter- 

 costal arteries. The 2nd intercostal artery may arise directly from the 

 trunk of the dorsal. External to the chest the dorsal artery is dis- 

 tributed to the parts beneath the scapula, and to the upper part of the 

 neck. 



The Superior Cervical Artery arises a little in advance of the pre- 

 ceding. It crosses the oesophagus and longus colli, and perforates the 

 upper end of the 1st intercostal space. It supplies the 1st intercostal 

 artery ; and, external to the chest, it is distributed in the neck. 



The Vertebral Artery has its origin a little in front of the preceding. 

 Passing obliquely upwards and forwards, it crosses the inner side of the 

 1st rib near its upper extremity, and enters the root of the neck to pass 

 in succession through the series of vertebral foramina. 



The Internal Thoracic Artery arises at the inner side or hinder 

 edge of the 1st rib. It descends along the inner face of the rib to the 

 floor of the chest, where it will be followed at a lower stage. 



The Inferior Cervical and External Thoracic Arteries are given 

 off from the axillary at the anterior edge of the 1st rib, and their roots 

 may be seen now, but they are distributed to parts without the thorax. 



The Anterior Vena Cava. This large vessel will be seen below the 



