578 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



downward and forward (Fig. 249). When the external intercostal 

 muscles, therefore, shorten, they tend, of course, to approximate their 

 origins and insertions, and so bring the ribs into a position in which 

 the distance between their point of origin and insertion will be the 

 shortest possible. This condition is, of course, fulfilled when the ribs 

 are horizontal ; therefore, the contraction of the external intercostal 

 muscles serves to elevate the ribs, and these muscles are, therefore, to 

 that extent inspiratory. 



In quiet inspiration the cavity of the thorax is increased by the 

 contractions of the scalene muscles, the levatores costarum, and external 

 intercostal muscles, which all serve to elevate the ribs. The diaphragm 

 also, by its contraction, is perhaps the most important inspiratory 

 muscle. 



If the head be fixed, the sterno-mastoid muscle, by its contraction, 

 serves to elevate the sternum, and, as a consequence, elevates all the 

 ribs. It, therefore, may be regarded as an accessory muscle of inspiration, 

 which is, however, only r employed in forced inspiration. If the scapula 

 be fixed, the pectoralis minor muscle, which rises from the coracoid 

 process of this bone to be inserted into the anterior extremities of the 

 upper ribs, will by its contraction also serve to elevate the ribs ; so, 

 also, the seratus posticus superior muscle may also, in forced inspiration, 

 serve to elevate the ribs, and so act as an auxiliary muscle of inspiration. 



The Mechanism of Expiration. — In the production of the enlarge- 

 ment of the chest, such as is essential to the accomplishment of inspira- 

 tion, it has been mentioned that the elasticity of the lungs and thoracic 

 walls has to be overcome by exertion of muscular force ; when the 

 inspiratory muscles relax, the elasticity of the lungs and thorax is alone 

 sufficient to cause the lungs to return to their original volume. This 

 contraction of the lungs, of course, occasions the expulsion of a quantity 

 of air from their interior, and expiration is therefore produced. It is 

 thus seen that expiration is .mainly a passive movement, due to the 

 reaction of the forces which have to be overcome in the production 

 of inspiration. 



It has been stated that when the diaphragm descends in inspiration, 

 by forcing the abdominal contents downward the abdominal walls are 

 put upon the stretch ; when the diaphragm relaxes the abdominal walls 

 tend to regain their original position, the abdominal organs are forced 

 upward into the thoracic cavity, and the borders of the ribs, which had 

 been slightly elevated, are now depressed. It is probable that the 

 elasticity of these different organs is in quiet expiration entirely 

 sufficient to balance the displacement produced in quiet inspiration. 

 The thorax may, nevertheless, be also reduced in volume to a greater 

 degree than is possible by the means already described. The muscles 



