396 



THE MUSCLES 



space behind the thin posterior border of the sheet is filled by a thin niemln'ane 

 composed of fibres running in the same direction, and is called the internal 

 intercostal fascia. 



Nerve-supply. — Branches from the intcnvostal nerves, which are sui)])]icd to its 

 outer surface or are given off where the nerves are concealed in the interior of the 

 muscle. 



Action. — See below. 



Relations. — Superficially, the external intercostal muscles, and the intercostal 

 arteries and nerves; deeply, the triangularis sterni, infracostales, dia})hragm, and 

 the pleura. 



The action of the intercostal muscles generally is to ai^proximate the ril)s to one 

 another, and they are chiefly used in inspiration. The obliquity of their fibres en- 



Exterual 

 intercoatals 



Internal 

 intercostals 



Fig. 300. — The Intercostal Mt^scles. 



A 



External 

 intercostals 



Internal 

 intercostals 



Infra -eostales 



al)les them with a small contraction to i)roduce a greater approximation than if they 

 ran peri)en(licidarly between the ribs (page 287). Moreover, if the fibres were all 

 directed, like those of the outer sheet, downwards and forwards, there would be a 

 tendency for the lower ribs to be drawn backwards as well as upwards in inspiration; 

 and if, on the other hand all the fibres were directed downwards and ])ackwards like 

 those of the inner sheet, the tendency Avould be for the lower ribs to be drawn 

 forwards as well as u])wards in inspiration. The combined action of the two sets 

 produces the direct elevation of the ril:»s, the forward pull of the iimer sheet being 

 counteracted by tlie l)ackward ])ull of the outer sheet. If the external intercostals 

 were carried forwards as far as the sternum, they would tend to take the sternum 

 as a fixed ])oint and to de])ress the anterior extremities of the ril)s. A similar result 

 would follow the continuation ))ackwards of the internal intercostals to the vertel)ral 



