MECHANISM OF THE RESPIRATORY MOVEMENTS. 247 



smaller arches above them ; and the second, by the movement or rotation of the ribs 

 round an axis extending from their heads to the sternum, which everts the lower edge 

 of the ribs, and increases the width of their arch outwards. 3rd. The capacity of the 

 thorax, transversely and posteriorly, is increased by the elastic bending of the ribs, as 

 well as the opening of the angle between the ribs and their cartilages, produced by the 

 resistance of the sternum and weight of the limbs to the forward and upward motion 

 of the extremities of the cartilages, and by the inclination backwards given to the 

 middle ribs in their upward movement by the oblique direction of the plane of the 

 costo-transverse articular surfaces. It is further to be noticed, that any considerable 

 elevation of the lowest ribs, instead of increasing, would diminish the capacity of the 

 thorax by raising the diaphragm, and accordingly in inspiration those ribs are drawn 

 backwards and outwards rather than raised. Lastly, it may be remarked, that 

 extension of the vertebral column is an important agent in respiration, for when the 

 column is bent forwards, the ribs are pressed together in the concavity of the curve, 

 and, conversely, when the column is extended the ribs are separated. 



The Intercostal Muscles. The manner in which these muscles act has been a sub- 

 ject of controversy from an early time, and cannot be said to be yet thoroughly 

 understood. Among those who look upon the intercostal muscles as active in 

 respiration, all are agreed that the external muscles are elevators of the ribs, and 

 therefore muscles of inspiration. According to one view, defended by Haller, the 

 external and internal layers have a common action, the decussating fibres acting in 

 the direction of the diagonal between them ; while according to another view, that of 

 Hamberger, while the external intercostal muscles are admitted to be elevators, the 

 internal are held to be depressors of the ribs. A more recent modification of these 

 views, which is at present most generally adopted, is that maintained by Hutchinson, 

 viz., that the external intercostal muscles, and the parts of the internal intercostals 

 placed between the costal cartilages, elevate the ribs, and that the lateral portions of 

 the internal intercostals act as depressors. This view is founded upon a mathematical 

 demonstration, and may be illustrated mechanically by an apparatus showing that if 

 two parallel bars, free at one end and attached by joints to a fixed perpendicular at 

 the other, be united by oblique elastic bands, one set of which is directed downwards 

 and away from the fixed upright, while another is directed downwards and towards 

 the upright, the first set of bands will be shortened by the elevation of the bars 

 from an oblique to a horizontal position, and the second set lengthened ; whereas 

 the first set will be lengthened and the second shortened by depression of the bars 

 below the horizontal position. It may be objected, however, to this vieT, that 

 the ribs differ from the supposed bars in respect that they are not rigid, and are not 

 free at either end, but have the greatest extent of motion, at least in some instances, 

 in the middle of their arch, and in the living subject, the ribs, in their elevation, both 

 rotate upon their axis and bend upon themselves, instead of describing a simple 

 upward and downward movement like the bars. The deficiency of the external inter- 

 costal muscles in front and of the internal behind, in which situations they would 

 have acted as depressors, seems to point to some sort of combined action of the 

 muscles as elevators of the ribs. 



Among the more recent anatomical writers, Henle inclines to Haller's view; 

 Luschka refers to Budge's experiments on the muscles of living animals, as proving 

 that the internal intercostal muscles elevate the ribs (Budge, "Lehrbuch der Phy- 

 siologic des Menschen," Weimar, 1860, p. 79) ; and Cruveilhier, founding on the 

 experiments of Beau and Massiat, supports the very different but scarcely tenable 

 view, that the intercostal muscles are not essential agents either in elevating or 

 depressing the ribs, but only act as tensors of the intercostal spaces (op. cit. p. 575). 



The levatores costarum have a similar action with the posterior fibres of the 

 external intercostal muscles, and ought therefore to be ranked among the agents of 

 inspiration. The scalene muscles also are usually believed to 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 posticus 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 expanded vertically. The fibres 



