BOUNDAKIES AND SIZE. 437 



section, the cavity is seen to be diminished in the middle line by the 

 prominent spinal column, on each side of which it projects backwards. 



Boundaries. On the sides are the ribs with the intercostal muscles ; Boundaries, 

 in front is the sternum ; and behind is the spine. 



The base is constructed at the circumference by the last dorsal The 

 vertebra behind, by the end of the sternum in front, and by the ribs 

 with their cartilages on each side ; while the space included by the 

 bones is closed by the diaphragm. 



The base is wider transversely than from before backwards, and form of 

 the diaphragm is convex upwards towards the chest ; though at surface> 

 certain spots it projects more than at others. Thus in the centre 

 it is slightly lower than on each side, and is on a level with the 

 base of the ensiform process. On the right side, forming a dome 

 over the liver, it rises to a level with the upper border of the fifth and height: 

 rib near its junction with the cartilage ; and on the left it arches 

 over the stomach to the corresponding part of the upper border of 

 the sixth rib. From the lateral projections, the diaphragm slopes 

 suddenly towards its attachment to the ribs, but more behind than 

 before, so as to leave an angular interval between it and the wall of 

 the chest. The level of the attachment of the diaphragm will be its side 

 marked by an oblique line, over the side of the chest, from the base 

 of the ensiform process to the eleventh dorsal spine ; but it differs 

 slightly on the two sides, being rather lower on the left. 



The apex of the thoracic cavity is continued higher than the Apex 

 osseous boundary, and reaches into the root of the neck. Its highest 

 point is not in the middle line, for there the windpipe, oeso- 

 phagus, blood-vessels and other structures lie, but it is prolonged on 

 each side for one or two inches above the anterior end of the first 

 rib, so that the apex may be said to be bifid. Each point projects is bifid: 

 between the scaleni muscles, and under the subclavian blood-vessels ; how 

 and in the interval between them lie the several objects passing ** 

 between the neck and the thorax. 



Dimensions. The extent of the thoracic cavity does not correspond Exteriorsiz* 

 with the apparent size externally ; for a part of the space included 

 by the ribs below is occupied by the abdominal viscera ; and the 

 cavity reaches upwards, as just stated, into the neck. 



In consequence of the arched condition of the diaphragm, the Depth 

 depth of the space varies greatly at different parts. At the centre, Aa 

 where the depth is least, it measures generally from six to seven before > 

 inches, but at the back about half as much again ; and the other 

 vertical measurements may be estimated by means of the data given or 

 as to the level of the attachment of the diaphragm on the wall of 

 the thorax. 



Alterations in capacity. The size of the thoracic cavity is con- Size is 

 stantly varying during life with the condition of the ribs and dia- SieT* 1 

 phragm in breathing. 



The horizontal measurements are increased in inspiration, when transversely 

 the ribs are raised and separated from one another, and are diminished nints of" 

 in expiration as the ribs approach and the sternum sinks. 



An alteration in depth is due to the condition of the diaphragm JH d T tl J r m y 



