MUSCLES OF INSPIRATION. 127 



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 attach- 

 ments of the intercostals are approximated. The ribs, from an excessively 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 short- 

 ened. If, by separating and elevating the ribs, the muscles be shortened, shortening of 

 the muscles will necessarily elevate and separate the ribs. In the three superior inter- 

 spaces, 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 upon the ribs from the 

 sternum, as the others do from the spinal column. The superior interspace is narrowed, 

 and the remainder are widened, in inspiration. 



Levatores Costarum. The action of these muscles cannot be mistaken. 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. They are more developed in man than in the 

 inferior animals. 



Auxiliary Muscles of Inspiration. The muscles which have just been considered are 

 competent to increase the capacity of the thorax sufficiently in ordinary respiration; 

 there are certain muscles, however, which are attached to the chest and the upper part 

 of the spinal column, or upper extremities, which may act in inspiration, although ordi- 

 narily 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 physiological, as after exercise, certain of them 

 (the ordinary auxiliaries) act for a time, until the tranquillity of the movements is 

 restored. But when there is obstruction in the respiratory passages or when respiration 

 is excessively difficult from any cause, threatening suffocation, all the muscles which can 

 by any possibility raise the chest are brought into action. The principal ones are put 

 down in the table under the head of extraordinary auxiliaries. Most of these muscles 

 can voluntarily be brought into play to raise the chest, and the mechanism of their 

 action can in this way be demonstrated. 



Serratus Posticus Superior. This muscle arises from the ligamentum nuchae, the 

 spinous processes of the last cervical and the upper two or three dorsal vertebra?, its fibres 

 passing obliquely downward and outward, to be attached to the upper borders of the 

 second, third, fourth, and fifth ribs just beyond their angles. By reversing its action, as 

 we have reversed the description of its origin and insertions, it is capable of increasing 

 the capacity of the thorax. 



Sterno-mastoideus. That portion of the muscle which is attached to the mastoid 

 process of the temporal bone and the sternum, when the head is fixed, is capable of act- 

 ing as a muscle of inspiration. It does not act in ordinary respiration, but its contrac- 

 tions can be readily observed whenever respiration is hurried or exaggerated. 



The following muscles, as a rule, act as muscles of inspiration only when respiration 

 is exceedingly difficult or labored. In certain cases of capillary bronchitis, for example, 

 the anxious expression of the countenance betrays the sense of impending suffocation ; 

 the head is thrown back and fixed; the shoulders are braced; and every available muscle 

 in brought into action to raise the walls of the thorax. 1 



1 Under these circumstances, some muscles which we have not thought it necessary to enumerate may act in- 

 directly as muscles of inspiration. 



