VISCERAL ANATOMY. 139 



side which represents the middle lobe of man, and also a notch 

 for the posterior cava. The apex or summit lies behind the first 

 rib and is somewhat detached, forming the anterior lobe. Thus 

 the right lung has three lobes, an anterior, middle and posterior, 

 the left two, an anterior and posterior. The superior border lies 

 near the vertebrae; the inferior is notched deeply for the heart. 



Structure. 



Serous from pleura. (See below.) 



Parenchyma. This is rose-colored, soft, strong and elastic. 

 It is divided into lobules, each of which receives a small bronchus, 

 and is further subdivided into alveoli, spaces with which a num- 

 ber of smaller spaces communicate, the air cells. The walls of the 

 air cells have a thin basement membrane, covered with flattened 

 epithelium YGITU" t Wfrrr f an mcn m diameter. Between the air 

 cells is yellow elastic connective tissue. 



Vessels. 



The vessels are derived from the pulmonary and bronchial 

 arteries and the pulmonary vein. They form a plexus under the 

 walls of the air cells, which allows the passage of oxygen. 



Nerves. 



Tenth and sympathetic. 



THE PLEURAE. 



The pleurae are two shut serous sacs, which form the borders of 

 the mediastinae, or space in centre of the chest. 



Structure. 



Basement membrane covered by flattened epithelium and lubri- 

 cated by a serous fluid. 



Attachment. 



It is attached to the root of the lung, there giving off theliga- 

 mentum latum, then envelops the whole lung, forming the visceral 

 layer. It then passes to the chest wall, forming its costal portion, 

 to the diaphragm, and at its middle is in contact with that of the 

 opposite side, forming its mediastinal portion; encloses the heart, 

 and divides the chest into the anterior and posterior mediastina. 



Nerves. 



Diaphragmatic, intercostal, tenth and sympathetic. 



Vessels. 



Bronchial and intercostal, 



