RESPIRATION 397 



contain fluid. The lungs therefore are devoid of air (atelectatic), and 

 haying a specific gravity greater than water, readily sink when placed in this 

 fluid. The capacity of the thorax does not exceed the volume of the lungs 

 With the first inspiration, however, the thoracic walls take a new position. 

 The air at once rushes into the lungs and distends them. But as the 

 capacity of the thorax even at the end of the expiration is now greater than 

 the volume which the lungs would assume unless distended, there at once 

 arises the elastic recoil in the opposite direction, the condition which gives rise 

 to the negativity of the pressure in the thoracic cavity. It is also probable 

 that as the child develops, the thorax grows more rapidly than the lungs, 

 giving rise to a condition which would increase and accentuate the elastic 

 tension and thus increase the negativity of the intra-thoracic pressure. 



THE DYNAMIC CONDITION 



In the dynamic condition, the thorax and its contained organs undergo a 

 series of movements in consequence of which the relations among them 

 characteristic of the static condition, are temporarily changed. To these 

 movements the term respiratory has been given, as a result of which, the 

 ventilation of the lungs is accomplished. 



The Respiratory Movements. The respiratory movements consist of 

 an alternate increase and decrease in the capacity of the thorax, accompanied 

 by corresponding changes in the capacity of the lungs, the two movements 

 being known as inspiration and expiration respectively. During the increase 

 in the thoracic capacity, the air passively flows into the lungs; during the 

 decrease in the thoracic capacity, the air passively flows out, of the lungs. 

 In both movements the lungs play an entirely passive part, their movements 

 being determined by the pressure of air within them and by the outward 

 movement of the thoracic walls, with which they are in close contact. 



1. Inspiration is an active process, the result of muscle activity. 



2. Expiration is a passive process, the result mainly of the recoil of the 



elastic tissue of the walls of the thorax and abdomen and of the elastic 

 tissue of the lungs. 



In inspiration the thorax is enlarged in all its diameters: viz., vertical, 

 transverse, and antero-posterior. In expiration the thorax is diminished in 

 all its diameters as it returns to its former condition. 



Inspiratory Muscles. The muscles which from their origin, direction, 

 and insertion contribute to the enlargement or expansion of the thorax are 

 quite numerous, and include those muscles which enter into the formation 

 of the thoracic walls (intrinsic muscles), as well as certain muscles which, 

 having their origin elsewhere, are attached to the thoracic walls at different 

 points (extrinsic muscles), though the extent to which they are called into 

 activity depends on the necessity for either tranquil or energetic inspirations. 

 The gradations between a minimum and a maximum inspiration are very 

 slight, and it is difficult to state at what particular instant any given muscle 

 begins to act. It is customary, however, to divide the muscles into two groups : 

 (i) Those active in the average or ordinary inspirations, and (2) those active 

 in maximum or extraordinary inspirations. Among the muscles active 

 in ordinary inspirations may be mentioned the diaphragm, the intercostales 

 externi, the inter cartilaginei, the levatores costarum, the scaleni, and the ser- 



