38 MECHANISM OF BREATHING, 



of the last twelve ribs, to the rearmost extremity of the 

 breast-bone, and to the spine under the loins , while its 

 apex or centre projects forwards. When it contracts, it 

 tends to become flat, and thus enlarges the capacity of the 

 chest by pushing back the contents of the abdomen. Its 

 action, especially during forced breathing, is aided by that 

 of another muscle, which is attached by one end to the 

 last four neck vertebrae, and by the other to the first rib, 

 which It pulls forward on contracting, and m this manner 

 helps to increase the size of the chest. 



Air is taken into, and expelled from, the lungs by 

 means of the alternate increase and diminution of the 

 capacity of the chest. When the former act occurs, the 

 air contained in the lungs becomes rarefied, and con- 

 sequently the external air rushes in through the animal's 

 nostrils to restore the balance of pressure. When the 

 latter takes place, a portion of the air which is already in 

 the lungs is forced out. It is evident, therefore, that the 

 power of taking a large volume of air into the lungs at 

 each breath, is more dependent on the difference between 

 the respective capacities of the chest when expanded, and 

 contracted, than on the actual size of the chest itself 



The act of breathing is called respiration; that of ex- 

 pelling: air from the lungs, expiration; and that of draw- 

 mg \x. \r., tnspration. 



The chest is enlarged by the diaphragm, as we have 

 seen ; and also by muscles which pull the middle pieces 

 of the ribs forward, so as to make_ the cavity wider from 

 side to side, and consequently to increase its size. The 

 ribs are drawn forward by muscles which cover them, and 

 by others which are situated between the successive pairs 

 of ribs. The lower ends of some of the ribs are also 

 drawn up during the act of inspiration. On page 29, I 

 have remarked on the power the ribs have of pivotting 

 on their ends. I may point out that when the air which 

 is within the chest, becomes rarefied at the commencement 

 of the act of inspiration, the lungs themselves passively 



