276 



RESPIRATION 



Inspiration. The enlargement of the chest during inspiration is due to 

 muscular action, which brings about an increase in the size of the chest cavity 

 through the contraction of the inspiratory muscles, the role played by the 

 lungs being a passive one. The chest cavity is increased in its three axes, 

 the vertical, lateral, and antero-posterior diameters. The muscles engaged 

 in ordinary inspiration are: the diaphragma, the intercostales externi, and 

 the scaleni and levatores costarum. During forced inspiration every 

 muscle is brought into play which by its contraction tends to elevate the ribs 



and sternum or which w r ill fix points against 

 which these muscles can act. This includes 

 almost every muscle of the trunk and neck. 

 Changes in the vertical diameter are due, 

 first, to the contraction of the diaphragm. 

 This muscle has the shape of a flattened 

 dome, its highest point being the central 

 tendon. While passive, its lower portions 

 are in apposition with the chest walls, figure 

 228, 7. On contraction, the dome is pulled 

 downward and the lower portion is pulled 

 away from the chest walls, the downward 

 displacement varying from 6 to 12 mm. in 

 normal respiration, and in forced respira- 

 tion may amount to as much as 45 mm. 

 The tendency of the diaphragm to pull the 

 lower ribs and lower part of the sternum 



inward is counteracted by the outward pressure of the abdominal viscera, 

 and by the action of the quadrati lumbori, which by their attachment to 

 the last ribs fix these and, in case of deep inspiration, may even pull them 

 downward. The serrati postici inferiores also aid, being attached to the 

 four lower ribs. 



Changes in the lateral and antero-posterior diameters are effected by the 

 raising of the ribs, which are attached very obliquely to the spine and sternum. 

 The elevation of the ribs takes place both in front and at the sides the 

 hinder ends being prevented from performing any upward movement by 

 their pivot attachment to the spine. The movement of the front extremities 

 of the ribs is of necessity limited by an upward and forward movement of the 

 sternum to which they are attached, the movement being greater at the lower 

 end than at the upper end of the sternum. 



The axes of rotation in these movements are two: one corresponding 

 with a line drawn through the two articulations which the rib forms with 

 the spine, a, b, figure 230, and the other with a line drawn from one of these 

 (head of rib) to the sternum, A B, figure 230; the motion of the rib around 

 the latter axis being somewhat after the fashion of raising the handle of a 



FIG. 228. Schematic Repre- 

 sentation of Diaphragm. In ex- 

 piration (/), quiet inspiration 

 (II), and deep inspiration (III). 

 (After Schaffer.) 



