MUSCLES OF INSPIRATION. 121 



movable, the direction of the fibres of the intercostals from above downward 

 and forward renders elevation of the ribs a necessity of their contraction, if 

 it can be assumed that the first rib is fixed or at least does not move down- 

 ward. The scalene muscles elevate the first rib in ordinary inspiration ; and 

 in deep inspiration, this takes place to such an extent as to palpably carry 

 with it the sternum and the lower ribs. Theoretically, then, th'e external in- 

 tercostals can do nothing but render the ribs more nearly horizontal. 



If the external intercostals be exposed in the dog in which the costal 

 type of respiration is very marked close observation can hardly fail to show 

 that these muscles enter into action in inspiration. If attention be directed 

 to the sternal portion of the internal intercostals, situated between the costal 

 cartilages, their fibres having a direction from above downward and back- 

 ward, 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 inspira- 

 tion 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 up- 

 per 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 position, are 

 rendered nearly horizontal ; and consequently the inferior attachments of the 

 intercostals are brought nearer the spinal column, while the superior attach- 

 ments 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 be shortened, it follows that shortening of the muscles will necessa- 

 rily elevate and separate the ribs. In the three superior interspaces, the con- 

 stant direction of the ribs is nearly horizontal, and the course of the .inter- 

 costal 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 upon the ribs from the sternum, as the others do from the spinal column. 

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



Levatores Costarum. The action of these muscles can not be mistaken. 

 They have immovable points of origin, the transverse processes of twelve 

 vertebras 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 Muscles 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 attached 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 



