A STUDY OF RESPIRATION 



217 



posterior. We shall now consider the provisions of struc- 

 ture that make this possible. 



Movements of the Ribs. In our study of the skeleton, we 

 learned that a pair of ribs is joined to each of the twelve 

 dorsal vertebrae, and that ten 

 of these pairs are attached 

 to the breastbone by carti- 

 lage. Most of the ribs, espe- 

 cially the lower ones, do not 

 run on a level from the spinal 

 column to the breastbone ; 

 the ventral ends are consid- 

 erably lower than are the 

 ends connected with the 

 backbone. When we inspire, 

 the muscles that run from 

 the anterior part of the trunk 

 to the ribs contract, and so 

 the ventral ends of these 

 bones are pulled upward 

 toward a horizontal position. 



By this movement the breast- The dotted lines show the position of 

 , . ,, , the ribs and sternum at inspiration, 



bone is pushed ventrally and 



the ribs themselves press outward at the sides (see Fig. 105). 

 In this way the capacity of the chest cavity is increased 

 from side to side and from dorsal to ventral regions. 



Structure and Movements of the Diaphragm. When the dia- 

 phragm is relaxed, it forms a dome-shaped partition between 

 the heart and lungs in the chest cavity and the stomach and 

 liver in the cavity of the abdomen (see Fig. 98, D). At the 

 apex of this dome is a tendon formed of tough connective 

 tissue, from which sheets of voluntary muscle run outward 

 and posteriorly on all sides. The muscle fibers are attached 

 to the lower end of the sternum, to several of the lower 

 ribs, and to the lumbar vertebrae. 



When, during inspiration, the muscles of the diaphragm 



FIG. 105. Diagram to show the 

 Movements of the Ribs and Ster- 

 num in Inspiration. 



c = cartilages of ribs. 

 7-5, r-6, r? = 5th, 6th, and 7th ribs. 



s = breastbone or sternum. 

 . v = vertebral column. 



