MUSCLES WHICH RAISE THE RIBS IOI 



situated between the costal cartilages, their fibres having a direction from 

 above downward and backward, it is equally evident that they enter 

 into action with inspiration. By artificially inflating the lungs after 

 death, it is seen that when the lungs are filled with air, the fibres of 

 these muscles are shortened (Sibson). In inspiration the ribs are all 

 separated posteriorly ; but laterally and anteriorly, some are separated 

 (all below the fourth), and some are approximated (all above the fourth). 

 Thus all the interspaces, except the anterior portion of the upper three, 

 are widened in inspiration. Sibson has shown by inflation of the chest, 

 that although the ribs are separated from each other, the attachments 

 of the intercostals are approximated. The ribs, from an oblique posi- 

 tion, are rendered nearly horizontal ; and consequently the inferior 

 attachments of the intercostals are brought nearer the spinal column, 

 while the superior attachments to the upper borders of the ribs are 

 slightly removed from it. Thus these muscles are shortened. If, by 

 separating and elevating the ribs, the muscles are shortened, it follows 

 that shortening of the muscles will necessarily elevate and separate the 

 ribs. In the three superior interspaces, the constant direction of the 

 ribs is nearly horizontal, and the course of the intercostal fibres is not so 

 oblique as in those situated between the lower ribs. These spaces are 

 narrowed in inspiration. The muscles between the costal cartilages 

 have a direction opposite to that of the external intercostals and act 

 on the ribs from the sternum, as the others act from the spinal column. 

 The superior interspace is narrowed, and the others are widened in 

 inspiration. 



Levatores Costamm. The action of these muscles can not be mis- 

 taken. They have immovable points of origin, the transverse processes 

 of twelve vertebrae from the last cervical to the eleventh dorsal, and 

 spreading out like a fan, are attached to the upper edges of the ribs 

 between the tubercles and the angles. In inspiration they contract 

 and assist in the elevation of the ribs. 



Auxiliary Mtiscles of Inspiration. The muscles which have just 

 been considered are competent to increase the capacity of the thorax 

 sufficiently in ordinary respiration ; but there are certain muscles at- 

 tached to the chest and the upper part of the spinal column or the 

 upper extremities, which may act in inspiration, although ordinarily the 

 chest is the fixed point and they move the head, neck or arms. These 

 muscles are brought into action when the movements of respiration are 

 exaggerated. When this exaggeration is but slight and is physiological, 

 as after exercise, certain of the ordinary auxiliaries act for a time, until 

 the tranquillity of the movements is restored ; but when there is obstruc- 

 tion in the respiratory passages or when respiration is difficult from any 



