288 A TEXTBOOK OF PHYSIOLOGY 



The infundibula also vary in size in these different zones of the lung, 

 being largest in the subpleural zone, and smallest at the root of the 

 lung (Fig. 153). 



The two surfaces which are most expanded are the diaphrag- 

 matic and the sterno-costal, or ventro-lateral. In general, the apical 

 surfaces remain almost stationary. It is only when the lungs are well 

 ventilated that the parts most remote from these surfaces of direct 

 expansion are brought properly into action. In people of sedentary 

 habits, therefore, such parts of the lung fall into a condition of disuse, 

 and receive a poor supply of blood, with its immunizing properties. 

 This explains why phthisis so frequently attacks the apex of the lungs 

 first. 



When inspiration becomes forced, accessory muscles, such as the 

 scaleni, sterno-mastoid, trapezius, pectoral, rhomboid, and serratus 

 aiiticus muscles are brought into play. The arms are fixed, so that 

 the muscles passing from the thorax to the arms can come into play 

 on the thorax. A patient suffering from dyspnoaa sits up, and grasps 

 the arms of a chair. 



In regard to expiration, it is often stated that quiet expiration is 

 brought about by a passive collapse of the expanded lung, the thorax 

 following this recoil by virtue of its weight. It seems probable that 

 such a process is aided and made to work smoothly, even in quiet 

 expiration, by the contraction of the muscles antagonistic to those 

 concerned in inspiration. Such muscles are those of the abdominal 

 Avail, and possibly the internal intercostals. 



In forced expiration many muscles are called into play, such as 

 the serratus posticus inferior and the rectus, obliquus, and trans- 

 versus muscles of the abdominal wall. 



