468 



EESPIRATION 



current arriving in the ventricles is then propagated to the left phrenic 

 nerve, because this nerve lies in close contact with the cardiac apex. 

 Moreover, as the diaphragmatic muscle reacts more quickly than the 

 ventricular, the former is generally seen to twitch before the systole of 

 the ventricles has fully developed.^ 



The Action of the Intercostal Muscles. — The ribs are connected 

 with one another by two sets of muscles, known as the intercostals. 

 The external intercostals extend obliquely downward and forward, their 

 attachment upon the rib above being nearer the spinal column than 

 their insertion upon the rib below. The internal intercostals pass in 

 the opposite direction, their place of attachment upon the rib below 

 being nearer the spinal column than their insertion upon the rib above. 

 The action of these muscles may readily be deduced from the adjoining 

 schema (Fig. 243) if it is remembered that a contracting muscle causes 

 its point of insertion to move closer to its point of attachment. If 



S I 



Fig. 243. — Diagram Illustrating the Action of the Intercostal Muscles. 



S, sternum ; V, vertebral ; A and B two consecutive ribs ; EEi, external intercostal mus- 

 cle; JJ\, internal intercostal muscle. The contraction of the first raises the ribs (//) , while 

 the contraction of the second lowers them (III). The distance SV is now shortened. 



A and B represent two consecutive ribs, the posterior extremities 

 of which are movable upon the vertebral column, but relatively 

 immovable upon the sternum, the line E to Ei indicates the direction 

 of the external intercostal fibers and the line J to Ji that of the inter- 

 nal intercostal fibers (Fig. 243, /). If the first muscle is now made to 

 contract, the ribs assume the position shown in Fig. 243, II, because the 

 distance between the two ends of the different external intercostal fibers 

 has been shortened. This muscle, therefore, elevates the ribs and 

 is inspiratory in its action. If the second muscle is now made to 

 contract, J is brought nearer to Ji and the ribs are depressed (Fig. 

 243, III). The internal intercostals, therefore, are expiratory in their 

 function. 



A similar but more elaborate representation of the action of the intercostal 

 and intercartilaginous^ muscles has been given by Hamberger, in 1727, but the 



1 Pike, Am. Jour, of Physiol., xl, 1916, 433. 



^ The inusc. intercartilaginei constitute that part of the muse, intercostales 

 interni which is situated between the costal cartilages. 



