236 RESPIRATION. 



ultimate bronchial tubes. The whole extent of respiratory sur- 

 face in both lungs has been calculated by Lieberkiihn 1 at fourteen 

 hundred square feet. It is plainly impossible to make a precisely 

 accurate calculation of this extent; but there is every reason to 

 believe that the estimate adopted by Lieberkiihn, regarded as 

 approximative, is not by any means an exaggerated one. The 

 great multiplication of the minute pulmonary vesicles, and of the 

 partitions between them, must evidently increase to an extraor- 

 dinary degree the extent of surface over which the blood, spread 

 out in a thin layer, is exposed to the action of the air. These 

 anatomical conditions are, therefore, the most favorable for its rapid 

 and complete arterialization. 



EESPIRATORY MOVEMENTS OF THE CHEST. The air which is con- 

 tained in the pulmonary lobules and vesicles becomes rapidly vitiated 

 in the process of respiration, and requires therefore to be expelled 

 and replaced by a fresh supply. This exchange or renovation of 

 the air is effected by alternate movements of the chest, of expansion 

 and collapse, which are termed the " respiratory movements of the 

 chest." The expansion of the chest is effected by two sets of mus- 

 cles, viz., first, the diaphragm, and, second, the intercostals. While 

 the diaphragm is in a state of relaxation, it has the form of a vaulted 

 partition between the thorax and abdomen, the edges of which are 

 attached to the inferior extremity of the sternum, the inferior 

 costal cartilages, the borders of the lower ribs and the bodies of 

 the lumbar vertebra^ while its convexity rises high into the cavity 

 of the chest, as far as the level of the fifth rib. When the fibres 

 of the diaphragm contract, their curvature is necessarily dimi- 

 nished ; and they approximate a straight line, exactly in proportion 

 to the extent of their contraction. Consequently, the entire con- 

 vexity of the diaphragm is diminished in the same proportion, 

 and it descends toward the abdomen, enlarging the cavity of the 

 chest from above downward. (Fig. 71.) At the same time the inter- 

 costal muscles enlarge it in a lateral direction. For the ribs, arti- 

 culated behind with the bodies of the vertebra?, and joined in front 

 to the sternum by the flexible and elastic costal cartilages, are so 

 arranged that, in a position of rest, their convexities look obliquely 

 outward and downward. When the movement of inspiration is 

 about to commence, the first rib is fixed by the contraction of the 



1 In Simon's Chemistry of Man, Philada ed., 1846, p. 109. 



