THE THORAX. 213 



are formed by the confluence of the internal jugular veins and the subclavian 

 and the lymphatic ducts. (Fig. 150.) 



The left brachio-cephalic vein (Fig. i 50) is the one structure that complicates 

 relations at the root of the neck. This vein lies in the anterior mediastinal 

 space, behind the first piece of the sternum. Great care must be taken not to 

 rupture this vessel or its tributaries when you remove the sternum and subse- 

 quently dissect this region. It is formed by the confluence of the left internal 

 jugular vein, the thoracic duct, the left subclavian vein, the left superior intercos- 

 tal, and the inferior thyroid veins. It crosses (i) the left subclavian artery ; (2) 

 the left common carotid artery ; (3) the two pneumogastric nerves and the left 

 phrenic nerve ; (4) the innominate artery ; (5) the left recurrent laryngeal nerve ; 

 (6) the trachea ; (7) the oesophagus. 



The thoracic duct (Fig. i 50) in many cases is filled with blood, the valves 

 at its terminus not being able to resist the pressure, used in embalming the 

 dissecting material. The many cases reported by students of absence of this ves- 

 sel have no foundation in fact, but belong to a host of finds and absences which 

 may collectively be known as cases of first dissection. 



Develop now the origin from the aorta of the innominate, the left common 

 carotid, and left subclavian arteries. You have dissected the branches of these 

 structures in your work on the neck ; now review their relations. See the innom- 

 inate artery dividing behind the right sterno-clavicular articulation into the right 

 common carotid and right subclavian arteries. 



The thoracic part of the aorta has two divisions. The student will have 

 noticed that this structure is in both the middle and posterior mediastinal spaces. 

 You must remember that classification of structures in reference to specific 

 regions is at best a very arbitrary institution ; that a knowledge of anatomy con- 

 sists in a thorough acquaintance with organs wherever found, rather than in an 

 ability to recite instanter an artificial, arbitrary classification of structures found 

 in parte in a given region. 



1. The aortic arch three propositions: (i) It begins at second sterno- 

 chondral articulation on the right side ; (2) it ends at the fifth thoracic vertebra 

 on-left side ; (3) its branches are : coronary, innominate, left common carotid, left 

 subclavian arteries. 4 



2. The thoracic aorta three propositions : (i) It begins at the fifth thoracic 

 vertebra on left side. (2) It ends at the twelfth thoracic in the mid-line. (3) Its 

 branches are : (i) Pericardiac to the pericardium ; (2) cesophageal to the oesopha- 

 gus ; (3) bronchial to the bronchi, and lung-substance ; (4) the posterior medias- 

 tinal branches, to the glands and connective tissue ; (5) the intercostals, ten in 

 number, to the muscles and fasciae in the intercostal spaces and to the skin cov- 

 ering these structures. 



Practical Dissection Observations. (i) Note that the branches to the bronchi, 

 oesophagus, glands, and pericardium are given off from the anterior part of the 

 aorta, or some derivative of the same. (2) Note that the mediastinal glands, in 

 nearly every case, contain a fluid of inky-black color. This dark color is pro- 

 duced by absorption of anthracite particles. When enough has accumulated to 

 produce carbuncnlar disease of the lung, the patient has anthracosis. You must 

 make this practical distinction : Every lung you find will have carbonaceous pig- 

 mentations ; not every lung will have enlarged, inky-fluid-containing lymphatics 

 only some will show this condition. Some lungs will possess carbuncular 

 nodules and nodes ; these always have enlarged lymphatics, filled with inky fluid, 

 if the disease is anthracosis. 



Conclusion. In every adult cadaver you will see pigmcnted lungs. In 

 some cadavers you may see anthracotic lungs that is, chronic interstitial pneu- 

 monia, produced by inhalation of atmospheric impurities, the basis of which is 

 carbon. 



