THE MECHANICS OF RESPIRATION 341 



ternal intercostals have the mastery and cause the costal borders to 

 spread. When the arch of the diaphragm is depressed, as in pleurisy 

 with effusion, emphysema, and empyema, the line of traction and the 

 line of the muscular fibers of the diaphragm correspond more closely, 

 so that the diaphragm is able to use its full force against the intercostal 

 muscles, with the result that the costal border moves towards the median 

 line. The curves of the different fibers of the diaphragm vary greatly; 

 the arch is much less marked in the portion attached to the costal margin 

 near the median line than in that attached in the axillary line. For this 

 reason the anterolateral part of the diaphragm requires less depression 

 to give it a horizontal position than is required for parts occupying a 

 more lateral position. A small pericardial effusion or an increase in the 

 size of the heart may therefore depress the diaphragm sufficiently to give 

 it mastery over the intercostals in the front portion, so that the costal 

 border may here move towards the midline, while the lower borders 

 move in a perfectly normal manner (see Figs. 116 and 117). 



During forced breathing several muscles are brought into play, among 

 the most important of which are the scaleni, sternomastoid, trapezius, 

 pectorals, rhomboids, and serratus magnus. 



There has been considerable debate as to whether expiration is normally 

 an active or a passive process. Undoubtedly the expiratory phase under 

 normal conditions does not require the same muscular effort as does that 

 of inspiration, but there are many observations which indicate that ex- 

 piration is partly under muscular control. The abdominal musculature, 

 for example, increases in tone during expiration, so as to bring about a 

 rise in the abdominal pressure, with the result that the relaxed diaphragm 

 is pushed up into the thoracic cavity. To this extent at least, expiration 

 is accompanied by increased muscular activity. 



Before leaving the subject of the diaphragmatic movements, reference 

 must be made to the recent observations of Lee, Guenther and Meleney 3 

 bearing on the general physiological properties of the diaphragmatic 

 muscle. They point out that most skeletal muscles in the living body 

 contract with varying degrees of intensity and at irregular intervals, 

 between which relatively long periods of rest occur, but the diaphragm 

 from birth to death performs a continuous succession of brief contrac- 

 tions of fairly regular rhythm and uniform extent, alternating with brief 

 periods of rest. Its muscle fibers, together with those of the other 

 respiratory muscles, therefore hold a unique position among skeletal 

 muscles, which suggests a crude analogy with that of the heart. They 

 have compared the physiological properties of the diaphragm with those 

 of the extensor longus digitorum, the sartorius, and the soleus, and found 



