404 TEXT-BOOK OF PHYSIOLOGY 



The scalenus muscles, anticus, medius, and posticus, arise from the trans- 

 verse processes of the cervical vertebrae, and after pursuing a downward and 

 forward direction are inserted into the sternal end of the first and second ribs. 

 The action of the first two, at least, is to elevate the first rib and thus establish 

 a fixed point from which the intercostal muscles can act. The posticus has 

 doubtless a similar action on the second rib. 



The serratus posticus superior, a quadrilateral sheet of muscle-fibers, 

 arises mainly from the spines of the last cervical and first and second thoracic 

 vertebrae. The anterior extremity is serrated and attached to the outer 

 surfaces of the second, third, fourth, and fifth ribs beyond the angle. The 

 action of the muscle is the elevation of the ribs to which it is attached. 



Forced Inspiration. In forced or extraordinary inspirations, whereby 

 the capacity of the thorax is still further increased, the foregoing muscles 

 are reinforced by the sterno-deido-mastoideus, the trapezius, and the pectorales 

 minor and major. Their functions will become apparent from a considera- 

 tion of their origins and insertions. 



Expiratory Forces and Muscles. Expiration, as previously stated, is 

 a passive process brought about by the recoil of the elastic tissues of the 

 thoracic and abdominal walls, and of the lungs, all of which have been 

 stretched and made tense during inspiration. With the cessation of the in- 

 spiratory effort the elastic forces, assisted by the weight of the ribs, sternum, 

 and soft tissues, return the thorax to its former condition. The result is a 

 diminution of all the diameters of the thorax. The vertical diameter is 

 diminished by the recoil of the tense abdominal walls, the replacement of the 

 abdominal organs and the consequent ascent of the diaphragm to its former 

 position. The transverse and antero -posterior diameters are diminished by 

 the descent of the ribs, sternum, and lungs. Coincident with the return of the 

 thoracic walls to their former condition there is a recoil of the elastic tissue 

 of the lungs, in consequence of which there is a compression of the intra- 

 pulmonic air. With its compression there is a rise of pressure above atmos- 

 pheric and at once there is an outflow of intra-pulmonic air until atmospheric 

 pressure is again established at the end of expiration. 



It is somewhat uncertain if a normal expiratory movement necessitates 

 active muscle contraction. If, however, there is any impairment of the 

 elasticity of the lungs or ribs, or any interference with the free exit of the 

 intra pulmonic air, it is highly probable that the elastic forces are assisted 

 by the internal intercostal and triangularis sterni muscles. It has been in- 

 sisted upon also that while the recoil of the elastic tissues is effective in the 

 early stages of an expiration, it is ineffective in the later stages. Hence there 

 arises a necessity for muscle assistance. 



The action of the internal intercostals is less clearly understood than that 

 of the external intercostals. If, however, the direction of these muscles as 

 indicated in Fig. 187, diagram A, by the dotted line ii, ii, be considered it 

 would seem that their action would be the opposite of that of the external 

 intercostals that is, it would be to depress the ribs. Employing the Ham- 

 berger model to elucidate the functions of these muscles it is apparent 

 that if the elastic band, ii, ii, recoils, the elastic force of the two halves, though 

 equal, will act in opposite directions, but as the component acting on the 

 long arm of the level preponderates over that acting on the short arm of 

 the lever, the ribs will be depressed. The action of the band is supposed 



