PULMONARY VESSELS. 331 



above the pulmonary veins. It is also invested, in common 

 with them, by that portion of the pleura which forms the 

 mediastinum, and thus enters into the composition of the root 

 of the lungs. 



The Pulmonary Veins are four in number — two on each 

 side. In conformity to the mode of description which we have 

 adopted, it may be said that they arise from the sides of the 

 Left Auricle, and proceed nearly in a transverse direction, two 

 of them to each lung ; where they accompany the branches of 

 the artery and of the trachea, being invested by the mediasti- 

 num, in common with these branches. It has been observed 

 that they differ from veins in general, by preserving a diameter 

 nearly similar to that of the arteries which they accompany. 



