RESPIRATION. 391 



acting upward on the long arm of the lever preponderates over the com- 

 ponent acting downward on the short arm of the lever. This taken in con- 

 nection with the fact that the distance between the adjoining bars is fixed, 

 leads not only to an elevation of the bars, but to a widening of the angle 

 between them and an advance of the second vertical support. The action 

 of these bands thus disclose and illustrate the action of both the external 

 intercostal and intercartilagenei muscles. It must therefore be concluded 

 that these muscles are the elevators of the ribs and cartilages and hence, 

 inspiratory in function. Of late the correctness of Hamberger's view has 

 been confirmed by experiments on living animals. 



The levatores costarum, as is evident from their points of origin and in- 

 sertion, elevate the ribs posteriorly. 



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 

 vertebras. 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. 



In forcible or extraordinary inspirations, whereby the capacity of the 

 thorax is still further increased, the foregoing muscles are reinforced by the 

 sterno-cleido-mastoideus , the trapezius, and the pectorales minor and major. 

 Their functions will become apparent from a consideration 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- 



