HUMAN PHYSIOLOGY 



section of the thoracic cavity is increased both in its anterior- 

 posterior and lateral diameter, as the horizontal distance between 

 the anterior ends of the ribs and the spinal column increases and 

 the lateral parts of the ribs move apart. Simultaneously with the 

 ribs, the sternum is raised and moved forward. The elevating of 

 the ribs and sternum is dependent upon the twisting of the 

 cartilage of the ribs. This increases the obtuse angle (facing 

 upward) formed by the cartilages of the ribs. 



The elevating of the ribs in quiet breathing is brought 

 about by the external intercostal and the intercartilaginous 

 portion of the internal intercostals. 



The fibres of the external intercostals, placed between the ribs, 

 slant forward and downward. As the ribs are raised, the insertion 

 points of any fibre approach each other, hence the contraction of 

 the muscle fibres elevates the ribs. 



The intercartilaginous portion of the internal intercostals slants 

 downward and backward between two costal cartilages which 

 slant in the same direction as the muscles. Here also the points 



I. 



*,, 



II. B 



W 



FIG. 2. 



of insertion approach each other when the cartilages are raised. 

 But these intercartilaginous muscles are of importance only in the 

 lower costal cartilages. 



In the above scheme (Fig. 2) W represents the spinal column; 

 B, the sternum ; R l and J? n represent two ribs with their external 

 intercostal muscle, mr; K v and A' u are two cartilages with their 



