300 VETEEINAEY PHYSIOLOGY 



fixed by the abdominal muscles. This greater movement is 

 simply due to the greater length of the muscles moving 

 the ribs. The muscles are chiefly the external intercostal 

 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 pulled forward, say, half an 

 inch. The first and second inter- 

 costals acting on the third rib 

 will together be two inches in 

 length, and in contracting they 

 can pull the third rib through, 

 say, half of two inches i.e. one 

 inch. The first, second, and third 

 intercostals, acting on the fourth 



rib, are three inches in length, 

 FIG. 137. Shows the Movements of 



the Ribs from their Position in and can therefore pull this rib 



Expiration to their Position in half of three, or one and a half, 



Inspiration. inches, and so through the other 



ribs, until the floating ribs fixed by the abdominal muscles are 

 reached. 



When the 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. 



Along with the intercostal muscles, the levatores costarum 

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

 the transverse and vertical diameters. 



These are the essential muscles of inspiration, but other 

 muscles also participate in the act. In many animals, even 

 when breathing quietly, it will be seen that the nostrils 

 dilate with each inspiration. This is due to the action of 

 the dilatores narium which contract synchronously with the 

 other muscles of inspiration. Again, if the larynx be examined, 

 it will be found that the vocal cords slightly diverge from 

 one another during inspiration. This is brought about by 

 the action of the posterior crico-arytenoid muscles (p. 335). 



Forced Inspiration. This comparatively small group of 

 muscles is sufficient to carry out the ordinary act of inspira- 

 tion. But, in certain conditions, inspiration becomes forced. 



