400 TEXT-BOOK OF PHYSIOLOGY 



this model v'-v' is a vertical support carrying two freely movable parallel 

 bars r r', united at their opposite ends with two other freely movable 

 and parallel bars cc, cc f carried by a second vertical support, s, representing 

 respectively the vertebral column, two adjoining ribs, two adjacent cartilages, 

 and the sternum. Diagram A shows the position of the different parts at 

 the end of expiration and B their position at the end of inspiration. The 

 parallel bars are joined to each other by elastic bands ei and ic having the 

 direction of and representing the external intercostal and intercartilaginei 

 muscles, respectively. The bars are depressed to sufficiently elongate and 

 tense the elastic bands and thus imitate the condition of the muscles in so 

 far as tension is concerned prior to their contraction. On releasing the 

 bars the elastic bands at once recoil and the bars representing ribs and 

 cartilages are raised. Although the elastic forces, acting in opposite direc- 

 tions as indicated by the arrows are equal, the bars are yet raised for the 

 reason that in accordance with the parallelogram of forces, the components 

 acting upward on the long arms of the levers preponderate over the com- 

 ponents acting downward on the short arms of the levers. This, taken in con- 

 nection with the fact that the distances between the adjoining bars are 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 actions 

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

 intercostal and intercartilaginei 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 

 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-cleido-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 



