306 DISSECTION OF THE THORAX. 



Thus in the centre it is lower than at the sides, and is on a level with the 

 base of the xiphoid cartilage. On the right side it rises to a level witli 

 the upper border of the fifth rib near the sternum ; and on the left to the 

 corresponding part of the upper border of the sixth rib. 1 From the lateral 

 projections, the diaphragm slopes suddenly towards its attachment to the 

 ribs, but more behind than before, so as to leave a narrow interval be- 

 tween it and the wall of the chest. The level of this attached part will 

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

 the xiphoid cartilage to the tenth rib ; but it differs slightly on the two 

 sides, being rather lower on the left (fig. 97). 



The apex of the space is continued higher than the osseous boundary, 

 and reaches into the root of the neck. Its highest point is not in the 

 middle line, for there the windpipe, bloodvessels, &c., lie ; but it is pro- 

 longed on each side for an inch or an inch and a half above the first rib, 

 so that the apex may be said to be bifid. Each point projects between 

 the scalerii muscles, and under the subclavian bloodvessels ; and in the 

 interval between them lie the several parts passing between the neck and 

 the thorax. 



Dimensions. The extent of the thoracic cavity does not correspond 

 with the apparent size externally ; for the space included by the ribs be- 

 low is occupied by the abdominal viscera, and the cavity reaches above 

 into the neck. 



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

 the space varies greatly at different points. At the centre, where the 

 depth is least, it measures about seven inches, but at the back as much 

 again ; and the other vertical measurements can be estimated by means of 

 the data given of the level of the base on the wall of the thorax. 



Alterations in capacity. The size of the thoracic cavity is constantly 

 varying during life with the condition of the ribs and diaphragm in 

 breathing. 



The horizontal measurements are increased in inspiration, when the ribs 

 are raised and separated from one another ; and are diminished in expi- 

 ration as the ribs approach and the sternum sinks. 



An alteration in depth is due to the condition of the diaphragm in res- 

 piration ; for the muscle descends when air is taken into the lungs, in- 

 creasing thus the cavity ; and ascends when the air is expelled from those 

 organs, so as to restore the previous size of the space, or to diminish it in 

 violent efforts. But the movement of the diaphragm is not equal through- 

 out, and some parts of the cavity will be increased more than others. For 

 instance, the central tendinous piece, which is joined to the heart-case, 

 moves but slightly ; but the lateral, bulging, fleshy valves descend freely, 

 and add greatly to the size of the lateral part of the chest by their separa- 

 tion from the thoracic parietes. 



The thoracic cavity may be diminished by the diaphragm being pushed 

 upwards by enlargement, either temporary or permanent, of the viscera 

 in the upper part of the abdomen ; or by the existence of fluid in the 

 latter cavity. 



1 This is the height in the dead body. The level to which it may reach in 

 great respiratory efforts during life will be stated with the account of the Dia- 

 phragm. 



