THE ELEVATORS OF THE RIBS. 



237 



are divided, death occurs (Budge, Eulenkamp) . The phrenic nerve contains some 

 sensory fibres for the pleura, pericardium, and a portion of the diaphragm 

 (Schreiber, Henle, Schwalbe). 



The contraction of the diaphragm is not to be regarded as a " simple muscular 

 contraction," since it lasts 4 to 8 times longer than a simple contraction ; it is rather 

 a short tetanic contraction, which we may arrest at any stage of its activity without 

 bringing into action any antagonistic muscles (Kronecker and Marckwald). 



(2.) The Elevators Of the Bibs. The ribs at their vertebral ends (which lie 

 much higher than their sternal ends) are united by means of joints by their heads 

 and tubercles to the bodies and transverse processes of the vertebra. A horizontal 

 axis can be drawn through both joints, around which the ribs can rotate upwards 

 and downwards. If the axes of rotation of each pair of ribs be prolonged on both 

 sides until they meet in the middle line, the angles so formed are greatest above 

 (125), and smaller below (88) (A. W. Volkmann). Owing to the ribs being 

 curved, we can imagine a plane which, in the passive (expiratory) condition of the 

 chest, has a slope from behind and inwards to the front and outwards. If the 

 ribs move on their axis of rotation this plane becomes more horizontal, and the 

 thoracic cavity is increased in its transverse diameter. As the axis of rotation of 

 the upper ribs runs in a more frontal, and that of the lower ribs in a more sagittal, 

 direction, the elevation of the upper ribs causes a greater increase from before 

 backwards, and the lower ribs from within outwards (as the movements of ribs 

 which are directed downwards are vertical to the axis). The costal cartilages 

 undergo a slight tension at the same time, which brings their elasticity into play. 



f 



Fig. 106. 

 Scheme of the action of the intercostal muscles. 



Changes in the Chest. All " inspiratory muscles" which act directly 

 upon the chest-wall, do so by raising the ribs; (a.) When the ribs are 



