142 ELEMENTARY PHYSIOLOGY less. 



the band being so short as to be put on the stretch when 

 hooked on to either x y, or w z, with the bars in the 

 horizontal position, A, the elasticity of the band, when 

 hooked on to x and y, will bring them up as shown in B ; 

 while, if hooked on to w and z, it will bring them down as 

 shown in C. 



Substitute the contractility of the external and internal 

 intercostal muscles for the shortening of the band, in 

 virtue of its elasticity, and the model will exemplify the 

 action of these muscles ; the external intercostals in 

 shortening will tend to raise, and the internal intercostals 

 to depress, the bony ribs. 



Such a model, however, does not accurately represent 

 tlie ribs, with their numerous and peculiar curves, and 

 hence, while all are agreed that the external intercostals 

 raise the ribs, the action of the internal intercostals is not 

 by any means so certain. 



The raising of the ribs which results from the action of 

 the external intercostal muscles is further assisted by the 

 contraction of the levatores costarum and perhaps 

 certain other muscles. The levatores costarum are 

 attached by their upper ends to the transverse pro- 

 cesses of the last cervical and first eleven dorsal ver- 

 terbraj, and each muscle is fastened by its lower end to 

 the rib next below the vertebra from whicli the muscle 

 itself springs. These muscles must also by their con- 

 traction raise the ribs. 



By means of these several muscles the ribs can be raised 

 from their naturally downward-slanting position into one 

 more nearly horizontal. When this takes place, the front 

 ends of the ribs must move not only upwards but for- 

 wards, and must therefore thrust the sternum slightly 

 outwards, or aAvay from the vertebral column. By this 

 movement the size of the thorax is of course increased 

 from back to front, an increase wliich may be easily felt by 

 placing one hand on the back and one on the chest of a 



