Inferior Thyroid Veins 187 



The tributaries of the innominate veins are the vertebral, internal 

 mammary, and inferior thyroid ; in addition, the left vein receives the 

 left superior intercostal, which passes obliquely to it over the front of 

 the transverse aorta. 



The inferior thyroid veins descend in front of the trachea, on either 

 side of the middle line ; the left ends in the left innominate, but the 

 right slopes over the front of the arteria innominata, to end at the con- 

 fluence of the innominate veins. These thyroid veins are important 

 in tracheotomy below the isthmus, and in ligation of the innominate 

 artery (p. 182). 



The superior vena cava, 3 in., is formed by the junction of the 

 innominate veins, behind the first right chondro-sternal joint. 



Course and relations. It descends by the right side of the ascend- 

 ing aorta, behind the inner end of the first and second intercostal 

 spaces, to the right auricle. Just at its beginning it lies on the right 

 side of the innominate artery. To its outer side and in front are 

 pleura and lung. Behind it is the root of the right lung, over which 

 the vena azygos major is hooking to enter the vena cava (v. p. 185). 



THE PLEURA 



The pleura consists of an external fibrous and an internal serous 

 layer. It is a large lymph-space, and com- 

 municates by stomata with the adjacent 



lymph- vessels. By its outer surface it ad- ^^g^ / J|X 

 heres to the chest-wall, diaphragm, and peri- 

 cardium ; it surrounds the lung, passing in 

 between the lobes. The interval between 

 right and left pleurae is divided into the me- 

 diastina (p. 154). 



There is actually no cavity between the 

 parietal and visceral layers, but, with a pene- 

 trating wound of the chest, or with rupture of 

 the lung, or with a fistulous opening from a 

 bronchus or cavity, air enters the pleural sac ; 

 the lung then collapses on account of the 

 elasticity of its wall, and that side of the chest 

 becomes tympanitic, and ceases to move in 

 respiration. This condition often follows 

 compound fracture of a rib. 



The lower border of the pleura is marked 

 by a line passing obliquely from the costo- 

 xiphoid articulation to the vertebral end of 

 the twelfth rib, but the border of the lung does not descend quite so 

 far, not even in deep inspiration, nor does the pleura quite fill in the 



Showing crevice between cos- 

 tal and phrenic pleura ; pul- 

 monary pleura ; phrenic and 

 hepatic peritoneum ; ribs, ix 

 to xn. 



