CHAP. VI.] 



THE MUSCLES. 



67 



and in contracting flattens out and descends, the abdominal 

 viscera are pressed downwards, and the thorax is expanded 

 vertically. In normal and quiet expiration the diminution of 

 the capacity of the chest is 

 mainly due to the return of 

 the walls of the chest to the 

 condition of rest, in conse- 

 quence of their own elastic 

 reaction, and of the elasticity 

 and weight of the viscera dis- 

 placed by inspiration. In more 

 forcible acts of expiration, and 

 in efforts of expulsion from the 

 thoracic and abdominal cavi- 

 ties, all the muscles which tend 

 to depress the ribs, and those 

 which compress the abdominal 

 cavity, concur in powerful ac- 

 tion to empty the lungs, to fix 

 the trunk, and to expel the con- 

 tents of the abdominal viscera. 

 Thus the diaphragm is an ex- 

 pulsive as well as the chief 

 respiratory muscle of the body. 

 Muscles of the upper extrem- 

 ity. A certain number of 

 muscles situated superficially 

 on the trunk pass to the bones 

 of the shoulder and of the 

 arm, so as to attach the upper 

 limbs to the trunk. Of these, 

 the two superficial muscles we 

 have mentioned as covering the 

 back, the trapezius and latis- 

 simus dorsi, and the pectoral 



muscles Covering the front of FIG. 58. -MUSCLES OF ARM. 58, biceps; 

 . 59, triceps. 



the chest, are the chief. Ihe 



most prominent muscles found in the upper limbs are : - 



Deltoid. Triceps. Supinators. Extensors. 



Biceps. Pronators. Flexors. 



