THE MECHANICS OF RESPIRATION 



337 



debate, and can not be said to be definitely settled. The direction of the 

 fibers in the internal intercostals indicates that they are expiratory in 

 function, since they can not shorten in the inspiratory position; while, 

 on the other hand, the fibers of the external intercostals can not shorten 

 in the expiratory position, and hence must be considered inspiratory in 

 character (Fig. 113). In 1751 Hamberger showed that mechanically this 

 is the case, and gave the schema shown in Fig. 114. 



The function of the intercartilaginous muscles, however, must be 

 inspiratory, as is shown in Fig. 115. It is possible, however, that the 

 main function of both intercostal muscles is to regulate the tone of the 

 intercostal spaces and so prevent their suction inwards when the nega- 

 tive pressure in the thorax increases (i.e., suction becomes greater). 



Fig. 115. Schema to demonstrate that the function of the internal intercartilaginous intercos- 

 tals is identical with that of the external interosseous intercostals. 



The ribs and costal cartilage may be regarded as rods bent at the angles acd and bef, in 

 which the articular points c and e represent the symphysis between the bony and the cartilaginous 

 parts on which traction is made. During inspiration the fibers of the intercartilaginous muscles, 

 which have the direction gh, move the sternum df away from the vertebral column ab, like the 

 fibers of the external intercostals, which run in the direction kl. During this double action the 

 angles c and e must be decreased, because the muscles of the upper intercostal spaces work simul- 

 taneously, and the entire thorax is slightly elevated during inspiration. From this scheme it is 

 apparent that the external intercostals and the intercartilaginous muscles must be the same. (From 

 L,uciani's Human Physiology.) 



The Action of the Diaphragm 



The ascent of the ribs, while producing an increase in the antero- 

 posterior and transverse diameters of the thorax, would decrease 

 the vertical diameter if this were not counteracted by the fixation 

 of the lower ribs and the descent of the diaphragm. The periph- 

 eral edges of the diaphragm are attached behind to the lumbar vertebrae, 

 laterally to the lower edges of the six lower ribs and their cartilages, 

 and in front to the tip of the ensiform cartilage. The fibers converge to 



