368 RESPIRATION. 



attached to the upper border of the rib below, are the eleven ex- 

 ternal intercostal muscles, the fibres of which have an oblique 

 direction from above downwards and forwards. Attached to 

 the inner borders of the ribs are the internal intercostals, which 

 have a direction from above downwards and backwards, at 

 right angles to the fibres of the external intercostals. There 

 are also a number of muscles attached to the thorax and spine, 

 thorax and head, upper part of humerus, etc., which are 

 capable of elevating either the entire chest or the ribs. 

 These must act as muscles of inspiration when the attach- 

 ments to the thorax become the movable points. Some of 

 them are called into action during ordinary respiration ; 

 others act as auxiliaries when respiration is a little exagger- 

 ated, as after exercise, and are called ordinary auxiliaries 

 while others, which ordinarily have a different function, 

 are only brought into play when respiration is excessively 

 difficult, and are called extraordinary auxiliaries. 



The following are the principal muscles concerned in in- 

 spiration : 



Muscles of Inspiration. 



ORDINARY RESPIRATION. 



Muscle. Attachments. 



Diaphragm Circumference of lower border of thorax. 



Scalenus Anticus Transverse processes of third, fourth, 



fifth, and sixth cervical vertebrae 



tubercle of first rib. 



Scalenus Medius Transverse processes of six lower cervi- 

 cal vertebrae upper surface of first 



rib. 



Scalenus Posticus Transverse processes of lower two or 



three cervical vertebrae outer sur- 

 face of second rib. 



External Intercostals Outer borders of the ribs. 



Sternal portion of Internal Intercostals . .Borders of the costal cartilages. 



Twelve Levatores Costarum Transverse processes of dorsal vertebras 



ribs, between the tubercles and 



angles. 



