VISCEPx-A OF THE CHEST. 211 



P). The junction of two pleiirre by means of cellular tis- 

 sue constitutes a mediastinum (see Fig 20) . Each side of 

 the chest has its distinct pleura, and the third for the heart 

 lies between the two. The third, however, is but very 

 small when compared with the other two ; consequently 

 there are spaces between the two pleurae where they meet, 

 and each such place is called a mediastinum. The medias- 

 tinum is of use in affording egress and ingress to various 

 tubes running to the viscera of the chest, or to that of the 

 abdomen, as well as to various glands necessary to nutri- 

 tion. The pleuree are serous membranes ; and as such 

 secrete a halitus or fluid, and it is their purpose to favour 

 motion. The pleura on each side of the chest being soft 

 and smooth can take various forms, just as a bladder when 

 fresh will cover the hand, and insert itself between the 

 fingers if pressed upon. The serous membrane on each 

 side of the chest is reflected over the insides of the ribs ; 

 thence on to the anterior of the diaphragm ; upon the ver- 

 tebrae and sternum ; hence ascending and descending they 

 form mediastinums, or are connected to one another by 

 cellular tissue. Then the lungs bulge out, and force the 

 inner side of the serous bag against the opposite side of the 

 sac, the two surfaces being in apposition ; for a serous mem- 

 brane, in its nature can only bear the touch of itself or its 

 secretion. The pleura reflected over the ribs is called the 

 costal pleura {Fig 20 . PC) ; that over the lungs the pul- 

 monary pleura {Fig 20 . PL) ; that which covers the front 

 of the diaphragm the phrenic pleura. The mediastinums 

 are four in number. The anterior and the superior ; the 

 middle and the inferior. The anterior contains veins, arte- 

 ries, nerve, and absorbents, going to and from the heart, 

 the trachea or windpipe, and the thymus gland or the 

 remains of it. The superior mediastinum contains the 

 vena azygos and the thoracic duct ; the posterior aorta and 

 the oesophagus ; the par vagum and sympathetic nerves. 

 The middle mediastinum contains only the heart and peri- 

 cardiac sac, or heart bag and its fluid. The inferior me - 

 diastinum holds nothing ; as the posterior vena cava is said 

 to pass through no division of this sort, but to have a fold 

 of jDleurato itself: a fact which anatomy corroborates. Of 

 the three serous sacs, that which holds the heart is very 



p2 



