THE RESPIRATORY MECHANISM. 389 



the result is that the lower rib is pulled up, and not the upper 

 down. In this way the lower ribs are raised much more than 

 the upper, for the whole external intercostal muscles on each 

 side may be regarded as one great muscle with many bellies, 

 each bell)'' separated from the next by a tendon, represented 

 by the rib. When the whole muscular sheet is fixed above 

 and contracts, it is clear that its lower end will be raised more 

 than any intermediate point, since there is a greater length 

 of contracting muscle above it. The elevation of the ribs 

 tends to diminish the vertical diameter of the chest; this is 



B 



FIG. 130. Portions of four ribs of a dog with the muscles between them, a, a, 

 ventral ends of the ribs, joining at c the rib cartilages, 6, which are fixed to carti- 

 laginous portions, d. of the sternum. A, external intercostal muscle, ceasing be- 

 tween the rib cartilages, where the internal intercostal, B. is seen. Between the 

 middle two ribs the external fntercostal muscle has been dissected away, so as to 

 display the internal which was covered by it. 



more than compensated for by the simultaneous descent of 

 the diaphragm. 



The Lateral Enlargement of the Chest is mainly due to 

 the diaphragm, which, when it contracts, adds to the lowest 

 and widest part of the conical chest cavity. Some small 

 widening is, however, brought about by a rotation of some of 

 the middle ribs which, as they are raised, roll round a little 

 at their vertebral articulations and twist their cartilages. 

 Each rib is curved and. if the bones be examined in their 

 natural position in a skeleton, it will be seen that the most 



