276 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



the first to the ninth ribs, an elevation of the ribs, by bringing, say, the 

 ninth rib into the position previously occupied by the eighth rib, must 

 cause an increase in the transverse diameter of the thorax at this 

 point. This increase in transverse diameter would be attended by 

 a corresponding shortening of the vertical diameter of the thorax, 

 but this shortening is more than counteracted by the fixation of the 

 lower ribs and the descent of the diaphragm. In the relaxed condition, 

 the diaphragm, in consequence of the pressure of the abdominal viscera 

 and elastic reaction of the abdominal muscles, is pressed upwards into 

 the thorax, presenting a convex surface to this cavity. Moreover, 

 the circumferential parts of the diaphragm come into close apposition 

 with the lower portion of the thoracic wall, the two layers of parietal 

 pleura being in contact. When the muscular fibres of the diaphragm 

 contract, two events occur, first, a lowering of the diaphragm ; and, 



second, a flattening of its circum- 

 ference, which is thus drawn away 

 from the chest wall, the space left 

 being filled by the descent of the 

 lung. Of these two results of the dia- 

 phragmatic contraction, the second 

 is by far the most important in 

 ordinary respiration. It may be 

 shown that, under these conditions, 

 the central tendon of the diaphragm 

 is practically motionless. As soon, 

 however, as the respiration becomes 

 laboured, there is an actual movement 

 downwards of the central tendon. 



The immobility of the central tendon 

 in normal respiration and its descent in 

 laboured inspiration can be shown in the 

 dog by passing a sound provided with a 

 FIG. 164. Diagrammatic representation of rubber capsule on the end down through 

 position of diaphragm during expiration the jugular vein into the inferior vena 



in^nrat^mT^^ 011 (H)> "^ ^^ Cava ' and then distendin g the capsule 



until it is held by the walls of the vessel. 



It will be seen that the projecting end of the sound only moves witli respiration 

 when the animal is rendered dyspnoeic in any manner. The difference in the 

 changes between the shape in the diaphragmatic floor of the thorax in normal 

 and laboured respiration is shown diagrammatically in Fig. 164. 



In consequence of the attachments of the diaphragm to the six lower 

 ribs, contraction of the diaphragm will tend to pull the ends of the upper 

 three of these downwards and inwards, of the lower three upwards and 

 inwards. Under normal circumstances, this tendency is counteracted by 

 the simultaneous expansion of the rib-cage effected by the inspiratory 

 muscles, and the fixation of the lower ribs by the serratus posticus 

 inferior, quadratus lumborum, and the lower portions of the sacro-lumbalis. 

 In cases of paralysis of the muscles which receive their innervation 

 from the cord below the origin of the phrenic nerves, and much more 

 frequently in children where the ribs are wanting in rigidity, the con- 

 traction of the diaphragm may cause a well-marked depression, running 

 downwards and outwards on each side in front of the chest, corre- 



