472 RESPIRATION. 



the thoracic walls. The elasticity of the ribs and their 

 cartilages, together with the mode of articulation between 

 the latter and the vertebrae, ah 1 tend to bring about such a 

 result. The action is, however, generally completed by 

 muscles, especially in deep expiration. The posterior ser- 

 rated muscle of the back, extending from the back and loins 

 downwards and forwards to the last six ribs, assists greatly 

 in the depression of these. The transversalis costarum, 

 extending from the first rib downwards and backwards to 

 the lateral parts of the sternum, compresses the first four 

 ribs. The triangularis sterni, attached to the upper surface of 

 the sternum, and the inner aspect of the cartilages of all the 

 true ribs, except the first, contracts the anterior part of the 

 thorax. This muscle is very active in carnivora, from the 

 great length of their costal cartilages. The external oblique, 

 which is attached to the fourteen last ribs, and the internal 

 oblique, which is fixed on the border of the last rib, actively 

 depress the posterior part of the thorax. The transversalis 

 abdommis, from its attachment to the cartilages of the false 

 ribs, acts along with the oblique. Lastly, the internal inter- 

 costals, which extend from the anterior border of one rib 

 downwards and forwards to the rib in front, are active 

 expiratory muscles, provided the last rib is a fixed point. 

 The thorax is diminished in an antero-posterior direction, 

 by the projection of the diaphragm into the chest during the 

 relaxation of that structure, and this is still further increased 

 by the contraction of the abdominal muscles. 



The great amount of elastic tissue in the bronchial tubes 

 and pulmonary tissue gives to the lungs an inherent contrac- 

 tile power of great energy, which comes into play in effecting 

 expiration. The muscular coats of the bronchia would 

 seem to act little, if any, in the regular rythmic breathing, 

 since, by watching their movements on the trachea, they 



