THE RESPIRATION 283 



ways the chest is increased from before backwards and from 

 side to side. 



The first pair of ribs does not undergo this movement; 

 the motion of the second pair of ribs is slight, but the 

 range of movement becomes greater and greater as we pass 

 downwards until the floating ribs are reached, and these are 

 fixed by the abdominal muscles. This greater movement is 

 simply due to the greater length of the muscles moving them. 

 The muscles moving the ribs are chiefly the external inter- 

 costal muscles, and these may be considered as acting from 

 the fixed first rib. Now if the 

 fibres of the first intercostal 

 muscle are one inch in length, 

 the second rib can be raised, 

 say, half an inch. The first and 

 second intercostals acting on the 

 third rib will together be two 

 inches in length, and in con- 

 tracting they can raise the third 

 rib through, say, half of two 

 inches i.e. one inch. The first, 

 second, and third intercostals, FIG. 133. Shows the Movements of 

 acting on the fourth rib, are tho Kibs from their Position in 



, i i i .-, i Expiration to their Position in 



three inches m length, and can Ration. 



therefore raise this rib half of 



three or one and a half inches, and so through the other 



ribs, until the floating ribs fixed by the abdominal muscles 



are reached. 



When tho diaphragm takes the chief part in inspiration 

 the breathing is said to be abdominal in type when the 

 intercostals chiefly act in raising the ribs it is said to be 

 thoracic. Abdominal breathing is best marked in males 

 thoracic in females. 



Along with the intercostal muscles, the levatores costarum 

 also act in raising the ribs and in increasing the thorax in 

 the transverse and antero-posterior diameters. 



These are the essential muscles of inspiration, but other 

 muscles also participate in the act. In many individuals, 

 even when breathing quietly, it will be seen that the nostrils 



