THE THORAX 955 



manner the three-Iobed condition of the adult right lung and the two-lobed 

 condition of the adult left lung are indicated. 



Each of these processes gives rise, by budding, to secondary processes, 

 and these in turn give rise, successively, to other processes. This budding 

 goes on very freely, and the ramifications constitute the palmonary lobes. 

 All the buds or processes, which carry along with them an investment of 

 mesoderm, give rise to the ramif^-ing system of bronchial tubes. The ter- 

 minal ramifications form the bronchioles and infandibula. The air-cells or 

 palmonary alveoli are formed as hollow sessile buds or evaginations of the 

 walls of the infundibula, with the cavities of which they communicate 

 freely. 



As stated, the epithelial cells of the lung-buds and of all their ramifications, 

 as well as the epithelial cells of the pulmonary diverticulum, are developed 

 from the entoderm of the fore-gut. 



Lungs of the Fcetus. — The lungs prior to birth, having been im- 

 pervious to air, feel solid, Uke Uver, and they would at once sink if 

 placed in a vessel containing water. 



Superior Mediastinal or Cardiac Glands. — These glands, which are 

 numerous and important, are situated in the superior mediastinum, 

 along the upper aspect of the arch of the aorta, in front of the lower 

 end of the trachea, and along the right and left innominate veins. 



They receive their afferent vessels from (i) the anterior mediastinal 

 glands, (2) the upper part of the pericardium, (3) the heart, and 

 (4) the thymus body, in early Hfe. Their efferent vessels terminate 

 in the thoracic duct and in the right lymphatic duct. 



Innominate Veins. — ^These vessels are two in number, right and 

 left, and each is formed by the junction of the internal jugular and 

 subclavian veins behind the inner end of the clavicle. They both 

 lie in the superior mediastinum. 



The right innominate vein is about an inch in length, and 

 passes downwards with a slight inclination inwards. At the level 

 of the lower border of the first right costal cartilage, close to the 

 sternum, it unites with the left innominate vein to form the superior 

 vena cava. Externally it is closely related to the right phrenic 

 nerve and right pleura ; its internal relation is the upper part of 

 the innominate artery ; and, behind it, is the right pnevmiogastric 

 nerve. 



The left innominate vein is about 3 inches in length, and passes 

 obliquely inwards and dowTiwards from left to right. As stated, 

 it joins its fellow of the right side to form the superior vena cava. 

 In front of it there are the upper part of the manubrium sterni, 

 the origins of the sterno-hyoid and sterno-th\Toid muscles, and 

 the remains of the thymus body. Behind it are the origins of the 

 innominate, left common carotid, and left subclavian arteries, the 

 left pneumogastric and left phrenic nerves, and two superficial 

 cardiac nerves from the cervical portions of the left pneumo- 

 gastric and left sympathetic. Below it there is the arch of the 

 aorta. 



There are no valves in the innominate veins. 



Tributaries. — Each vein receives the following tributaries : 

 (i) the vertebral vein ; (2) the inferior thyroid vein ; (3) the internal 



