646 PHYSIOLOGY OF RESPIRATION. 



thorax may return passively to its normal position, giving what is 

 known as a passive expiration, that is, an expiration not caused 

 by muscular effort. So after an active expiration the thorax may 

 return passively to its normal position, giving a passive inspiration. 

 Our normal respiratory movements consist of an active inspiration 

 followed by a passive expiration, 



Mechanism of the Inspiration. The chest cavity may be 

 enlarged and an inspiration, therefore, be produced by two methods, 

 namely, by a contraction of the diaphragm and by an elevation 

 of the ribs. 



Contraction of the Diaphragm. From the anatomy of the 

 diaphragm it is evident that its fixed attachment is found in its 

 muscular connections with the lumbar vertebrae, the ribs, and the 

 ensiform cartilage. From these attachments the muscular sheet 

 extends anteriorly along the walls of the thorax and then bends over 

 to form the arch which ends in the central tendon. This latter 

 structure is not entirely free, since it is attached to the pericar- 

 dium of the heart ; but, relatively, it is the movable portion of 

 the diaphragm. Speaking generally, a contraction of the dia- 

 phragmatic muscle draws the central tendon downward toward the 

 abdominal cavity and therefore enlarges the chest in the vertical 

 diameter, while an increase in the thoracic cavity around the 

 periphery of the diaphragm is caused also by the flattening of the 

 muscular arch. Two results follow this movement: The lungs are 

 expanded exactly in proportion as the cavity enlarges. There is> 

 of course, at no time any space between the lungs and the dia- 

 phragm : as the latter moves downward the lungs follow because of 

 the excess of pressure on their interior. Although ordinarily we 

 speak of the new air being sucked into the lungs during this move- 

 ment, it is, of course, strictly speaking, forced in by the pressure of 

 the outside atmosphere. On the other hand, the descent of the dia- 

 phragm raises the pressure in the abdominal cavity. This cavity is 

 entirely full of viscera and for mechanical purposes may be regarded 

 as being full of liquid. The rise of pressure is transmitted throughout 

 the abdomen and causes the abdominal wall to protrude. Inspiration 

 caused by a contraction of the diaphragm is therefore spoken of 

 either as diaphragmatic respiration or as abdominal respiration, the 

 latter term having reference to the visible effect on the abdominal 

 walls. In strong contractions of the diaphragm the heart also is 

 pulled downward, and if the movement is forced the lower ribs may 

 be pulled inward to some extent. This last effect would diminish 

 the size of the thorax and therefore would tend to antagonize the 

 inspiratory action of the diaphragm, and other muscles are appar- 

 ently brought into play to prevent this result. As stated below, the 



