354 THE RESPIRATORY SYSTEM 



passage of infection when the latter group is the site of 

 tuberculous disease (p. 352). 



The Movements of Respiration. In the healthy adult 

 male, the increase in the capacity of the thorax which is 

 necessary during inspiration is obtained by the descent of the 

 diaphragm, and, to a much lesser degree, by the contraction of 

 the intercostal muscles. With each inspiration the fleshy 

 fibres of the diaphragm, which are slightly arched, straighten 

 out and pull on the central tendon. As a result of the 

 straightening out of the fleshy fibres, the abdominal viscera 

 are pressed downwards and the relaxed muscles of the 

 abdominal wall are bulged in an outward direction. The 

 central tendon descends, but only to a very slight extent, the 

 amount of which may be gauged by placing the finger on the 

 thyreoid prominence during deep respiration. The movement 

 is transmitted from the central tendon to the fibrous peri- 

 cardium, and from the fibrous pericardium to the pretracheal 

 fascia. Expiration is brought about by the recoil of the 

 muscular abdominal wall, which presses the abdominal viscera 

 upwards against the diaphragm, causing it to resume its 

 rounded dome-like shape. This variety is known as the 

 abdominal or abdomino-thoracic type of respiration. 



In the adult female, the type of respiration is termed 

 thoracic or thoracico-abdominal. The effect of the diaphragm 

 is much less marked, and, to make up for this diminution in 

 vertical depth, the transverse and antero-posterior diameters 

 of the thorax are definitely increased during inspiration. This 

 result is obtained by the action of the intercostal muscles, 

 which raise the ribs "like pail handles." This movement in- 

 creases the transverse diameter of the thorax and it also 

 increases the antero-posterior diameter, as the sternal ex- 

 tremities of the ribs are thrust forwards and they carry the 

 sternum with them. The elevation of the eighth, ninth and 

 tenth ribs, which are not attached directly to the sternum, 

 causes an increase in the infra-costal angle, so that not only 

 is the transverse diameter of the thorax increased, but, as more 



