THE THORAX. 365 



THE PLEURA. 



The pleura is a serous membrane which covers the lung, pleura 

 pulmonalis or serous layer of the pleura, being at its root reflected 

 so as to line the thorax, forming the pleura costalis or parietal layer 

 of the pleura. The theoretical 'space between the two layers is 

 the pleural cavity. Inasmuch as these layers are normally always 

 in contact, there is no actual space or cavity between them, 

 although they are moistened by a small amount of secretion for 

 lubricating purposes. When fluid collects here, as it does in some 

 forms of pleuritis or inflammation of the pleura, the layers are 

 then separated. 



Blood-vessels. The arteries which supply the pleura have 

 their origin in the intercostal, internal mammary, musculophrenic, 

 thymic, pericardiac, and bronchial arteries. The veins are similar 

 in their anatomic relations. 



Nerves. The nerves are of phrenic and sympathetic origin, 

 and accompany the branches of the bronchial artery. 



THE THORAX. 



The thorax or chest is the structure which contains the lungs, 

 heart, and great blood-vessels. The thoracic cavity is the 

 space within the thorax in which these organs are located. The 

 thorax is formed by the vertebral column and the ribs posteriorly, 

 the sternum and the costal cartilages anteriorly, and by the ribs 

 laterally. The spaces between the ribs, intercostal spaces, are filled 

 by the intercostal muscles. These muscles and others which are 

 attached to the thorax are concerned in the movements of respira- 

 tion. 



Vertebral Column. This portion of the thorax is rigid and 

 takes no part in any of the movements connected with respiration. 

 The vertebrae which are concerned in the formation of the thorax 

 are the 12 dorsal (Figs. 206, 207), and the anatomic points of 

 interest are the facets and demifacets on their bodies, which form 

 articulating surfaces for the heads of the ribs, and the facets on the 

 transverse processes for articulation with the tubercles of the ribs. 

 Between the vertebrae are intervertebral disks of fibrocartilage, 

 which under the microscope present the appearance of fibrous 

 tissue with articular cartilage (Fig. 31, page 39). 



Ribs. All the ribs, from the first to the twelfth, enter into 

 the formation of the thorax, and articulate posteriorly with the 

 vertebrae. 



The first seven, the true ribs, are attached anteriorly to the ster- 

 num, not directly, but by means of the costal cartilages ; while of the 

 others, the false ribs, 3, the eighth, ninth, and tenth, are attached 

 to the cartilage of the seventh rib, and, 2, the eleventh and 

 twelfth, the floating ribs, are free at their anterior extremities. 



