342 RESPIRATION. 



sternal ends of the ribs. The external intercostals and the levatores costarum 

 \vith the scaleni may fairly be said to be the elevators of the ribs, i. e., the 

 chief muscles of costal inspiration in normal breathing. 



It must be added, however, that some observers deny that either set of 

 intercostal muscles take any important part in raising the ribs. They hold 

 that the chief if not the only use of these muscles is by their contraction to 

 render the intercostal spaces firm and the whole thoracic cage rigid, so that 

 the thorax is moved as a whole by the other muscles mentioned, and the 

 intercostal spaces do not give way during the respiratory movements. 



Additional space in the transverse diameter is afforded probably by the 

 rotation of the ribs on an antero-posterior axis ; but this movement is quite 

 subsidiary and unimportant. When the chest is at rest, the ribs are some- 

 what inclined with their lower borders directed inward as well as downward. 

 When they are drawn up by the action of the intercostal muscles, their 

 lower borders are everted. Thus their flat sides are presented to the thoracic 

 cavity, which is thereby slightly increased in width. 



277. Labored inspiration. When respiration becomes labored, other 

 muscles are brought into play. The scaleni are strongly contracted, so as 

 distinctly to raise or at least give a very fixed support to the first and second 

 ribs. In the same way the serratus posticus superior, which descends from 

 the fixed spine in the lower cervical and upper dorsal regions to the second, 

 third, fourth, and fifth ribs, by its contractions raises those ribs. In labored 

 breathing a function of the lower false ribs, not very noticeable in easy 

 breathing, comes into play. They are depressed, retracted, and fixed, thereby 

 giving increased support to the diaphragm, and directing the whole energies 

 of that muscle to the vertical enlargement of the chest. In this way the 

 serratus posticus inferior, which passes upward from the lumbar aponeurosis 

 to the last four ribs, by depressing and fixing those ribs becomes an adjuvant 

 inspiratory muscle. The quadratus lumborum and lower portions of the 

 sacro-lumbalis may have a similar function. 



All these muscles may come into action even in breathing which, though 

 deeper than usual, can hardly perhaps be called labored. When, however, 

 the need for greater inspiratory efforts becomes urgent, all the muscles which 

 can, from any fixed point, act in enlarging the chest, come into play. Thus 

 the arms and shoulder being fixed, the serratus magnus passing from the 

 scapula to the middle of the first eight or nine ribs, the pectoralis minor 

 passing from the coracoid to the front parts of the third, fourth, and fifth 

 ribs, the pectoralis major passing from the humerus to the costal cartilages, 

 from the second to the sixth, and that portion of the latissimus dorsi which 

 passes from the humerus to the last three ribs, all serve to elevate the ribs 

 and thus to enlarge the chest. The sterno-mastoid and other muscles passing 

 from the neck to the sternum, are also called into action. In fact, every 

 muscle which by its contraction can either elevate the ribs or contribute to 

 the fixed support of muscles which do elevate the ribs, such as the trapezius, 

 levator anguli scapulse, and rhomboidei by fixing the scapula, may, in the 

 inspiratory efforts which accompany dyspnoea, be brought into play. 



278. Expiration. In normal easy breathing, expiration is in the main 

 a simple effect of elastic reaction. By the inspiratory effort the elastic tissue 

 of the lungs is put on the stretch ; so long as the inspiratory muscles continue 

 contracting, the tissue remains stretched ; but directly those muscles relax, 

 the elasticity of the lungs comes into play and drives out a portion of the 

 air contained in them. Similarly the elastic sternum and costal cartilages 

 are by the elevation of the ribs put on the stretch ; they are driven into a 

 position which is unnatural to them. When the intercostal and other ele- 

 vator muscles cease to contract, the elasticity of the sternum and costal car- 



