GENERAL REMARKS UPON THE CHEST. 469 



behind. The superior opening of the chest presents ob- 

 liquely downward and forward, and is oval, its lateral 

 diameter being the greatest. It gives passage to th-3 

 trachea, oesophagus, vessels, nerves, muscles, cellular tis- 

 sue, &c. The base of this cavity is represented by the dia- 

 phragm, the circumference of which is bounded by the 

 xiphoid cartilage, the inferior margin of the cartilages of 

 the false ribs, and the lower dorsal vertebrae. 



The dimensions of the thoracic cavity vary according to 

 age and in different individuals, and in the same individual 

 according to the state of the diaphragm, whether ascending, 

 descending, or quiescent. The capacity of the chest is 

 intermediate between that of the abdominal and cranial 

 cavity. Its diameters are three, the antero-posterior, trans- 

 verse, and vertical. The antero-posterior has the greatest 

 length below, and is shorter at either end than on the 

 middle line, in consequence of the projection forward of 

 the bodies of the vertebrae. The transverse is longest across 

 the eighth ribs. The vertical has the greatest length of the 

 three, and is longer at the sides than the middle, in conse- 

 quence of the descent of the diaphragm. 



During inspiration all these, diameters are increased 

 and the capacity of the chest enlarged in every direction, 

 by the elevation of the ribs and the fall of the diaphragm. 

 In expiration, on the contrary, these diameters are all less- 

 ened by the falling of the ribs and the ascent of the dia- 

 phragm, and the expulsion of air from the chest. 



In the foatus these motions do not, of course, take place, 

 and the form of the chest is very different from that of the 

 adult. The thoracic cavity is short, its sides compressed, and 

 its base very broad, in consequence of the collapsed state of 

 the lungs, and the great size of the liver. 



The antero-posterior diameter is large, to provide for the 

 heart and thymus gland, while on the sides it is compara- 

 tively small, on account of the unexpanded condition of the 

 lungs. The vertical diameter is less, the ribs are closer 

 together, and the intercostal spaces shorter. So soon, how- 

 ever, as respiration begins, the chest expands, and all the 

 peculiarities just mentioned are lost. 



