826 THE MUSCLES AND FASCIA OF THE TRUNK. 



the ribs differ from such rods in respect that they are not straight or rigid bars, and are not 

 free at either end, but are deeply curved, and have the greatest extent of motion in the 

 middle of their arch ; and in the living subject the costal arches, in their elevation, both 

 rotate upon certain axes and diminish their curvature, instead of describing a simple upward 

 and downward movement like the bars, so that it is impossible to draw any certain conclusion 

 from such imperfect imitations of the mechanism. The view advocated by Haller, that the 

 two muscles act in combination as elevators of the ribs, appears to derive support from 

 several circumstances, among which may be particularly mentioned the deficiency of the 

 external intercostal muscles in front and of the internal behind, in situations where both 

 would undoubtedly act as depressors, and the experiments of Duchenne, 1 who found that the 

 direct galvanic stimulation of the external intercostal muscle throughout the fifth space caused 

 the lower rib to rise towards the upper, and that the stimulation of the internal intercostal 

 muscle in the intercartilaginous part of the space also caused the lower rib to rise, and farther 

 that when the galvanic stimulus was so strong as to reach the intercostal nerve, and throw the 

 whole of the internal as well as the external intercostal muscle into action, the lower rib was 

 still elevated. On the other hand, the careful and extensive series of measurements carried 

 out by v. Ebner 2 on the partially dissected thorax, both in the collapsed and inflated condi- 

 tions, tend to confirm Hutchinson's view, indicating that, with the exception of the first two 

 intervals, the intercostal spaces are generally widened and the fibres of the interosseous internal 

 intercostals lengthened during inspiration. 



Apart from the action of these muscles in producing movements of the ribs, they also fulfil 

 an important function in supporting and maintaining an equable tension in the portions of 

 the thoracic wall corresponding to the intercostal spaces, without opposing the resistance to the 

 movements that would be presented by a firmer although elastic structure in the same situation. 



The Icvatores costarvm are usually considered to have a similar action with the posterior 

 fibres of the external intercostal muscles, and are therefore ranked among the agents of inspi- 

 ration. The above-mentioned experiments of v. Ebner, however, show that these muscles have 

 very little influence upon the elevation of the ribs, but that their fibres are most contracted 

 during extension and lateral flexion of the dorsal part of the spinal column. The seal cue 

 muscles also contribute, even in normal and quiet inspiration, to the support and elevation of 

 the first and second ribs ; and it is obvious that the serratus 2)< > $tic i us superior must have a 

 similar effect on those upper ribs to which it is attached. 



The action of the diaphragm is more easily understood than that of the intercostal muscles. 

 By its contraction and descent its convexity is diminished, the abdominal viscera are pressed 

 downwards, and the thorax is expanded vertically. The extent to which the central tendon 

 descends in full inspiration is estimated by Hasse 3 at one inch for the right lobe, four-fifths of 

 an inch for the left lobe, and two-fifths of an inch for the hinder part of the middle lobe. The 

 fibres arising from the ribs, being directed nearly vertically upwards from their origins, must 

 tend to raise those ribs, and Duchenne has shown 4 that .the contraction of the diaphragm by 

 itself elevates and expands the upper ribs to which it is attached, but only so long as the 

 vault of the muscle is supported by the abdominal viscera, for when they are removed it no 

 longer has that action. The serratus post'tcus inferior and quadratics lumborum muscles, by 

 opposing the diaphragm, and thus giving it a fixed point on which to descend, become assistant 

 muscles of inspiration. The anterior fibres of the diaphragm, being directed more horizontally 

 towards the central tendon, oppose the forward motion of the sternum ; hence the sternum 

 becomes arched in patients long subject to asthma. 



In more forcible inspiration, and more especially in severe attacks of dyspnoea, there are 

 called into play other powerful muscles, to secure the inspiratory action of which a fixed 

 attachment must be provided by the support and elevation of the shoulder and arm : among 

 these may be enumerated the pectoralis minor, the lower part of the pectoralis major, and pos- 

 sibly also the lowest slips of the serratus maynus. Additional assistance in elevating the 

 sternum and upper ribs is afforded by the stcrno-mastoid and the infrahyoid muscles. 



Expiration. In normal and quiet expiration the diminution of the capacity of the chest 

 is mainly, if not wholly, due to the return of the walls of the chest to the condition of rest, 

 in consequence of their own elastic reaction, and of the elasticity and weight of the viscera 

 and other parts displaced by inspiration ; the lungs themselves, after distension by air, exert 

 considerable elastic force, and no doubt the ribs and their cartilages react strongly by their 

 elastic return from the elevated and expanded condition into which they had been thrown by 

 the inspiratory forces. In more forcible acts of expiration, in muscular efforts of the limbs 

 and trunk, and in efforts of expulsion from the thoracic and abdominal cavities, all the muscles 

 which tend to depress the ribs, and those which compress the abdominal cavity, concur in 

 powerful action to empty the lungs, to fix the trunk, and to expel the contents of the abdo- 

 minal viscera. (See farther. " Action of the Abdominal Muscles.") 



1 "Physiol. des Mouvements, " &c., 1867, p. 646. - Archiv fur Anatomic, 1880, p. 185. 



3 Archiv fur Anatomic, 1886, p. 199. 4 Op. cit. p. 620. 



