THE THORAX 927 



point where the fibres cease the upper six muscles are continued 

 inwards to the side of the sternum by a thin delicate membrane, 

 called the anterior intercostal aponeurosis. This occupies the spaces 

 between the true costal cartilages. Posteriorly the muscles extend 

 as far as the tubercles of the ribs. 



Internal Intercostal Muscles — Origin. — The upper margin of the 

 subcostal groove of the upper rib. 



Insertion. — ^The inner margin of the upper border of the lower rib. 



Nerve-supply. — ^The intercostal nerve of the corresponding space. 



The fibres of each muscle are directed downwards and backwards, 

 and in this respect coincide with those of the obliquus intemus 

 abdominis. The upper six muscles extend as far inwards as the 

 side of the sternum, and the lower two are continuous anteriorly 

 with the obliquus internus abdominis. Posteriorly the muscles extend 

 as far back as the angles of the ribs. In this situation each is 

 replaced by a delicate membrane, called the posterior intercostal 

 aponeurosis, which lines the adjacent portion of the external inter- 

 costal muscle, and blends behind with the anterior or external 

 superior costo-transverse ligament. The internal intercostal muscles 

 are covered internally by the parietal pleura. 



Action of the Intercostal Muscles. — ^This subject has given rise to 

 much discussion and difference of opinion. Three views are enter- 

 tained, (i) According to Haller, the external and internal intercostal 

 muscles both act as elevators of the ribs, and are therefore muscles 

 of inspiration. Inasmuch as the fibres of the two muscles decus- 

 sate, they must, according to this view, act on the principle of the 

 parallelogram of forces. The common nerve-supply of the two 

 muscles tends to favour this view. (2) According to Hamberger, the 

 external intercostal muscles are elevators of the ribs, and therefore 

 muscles of inspiration, whilst the internal intercostal muscles are 

 depressors of the ribs, and therefore muscles of expiration, 

 (3) According to Hutchinson, the external intercostal muscles and 

 the inter chondral portions of the internal intercostal muscles act as 

 elevators of the ribs, and are therefore muscles of inspiration, whilst 

 the interosseous portions of the internal intercostal muscles act as 

 depressors of the ribs, and are therefore muscles of expiration. 



When a rib is elevated, its lower border is at the same time 

 everted. 



For the levatores costanim muscles, see Indax. 



Intercostal Nerves. — ^Tbese are eleven in number on each side, and 

 are the anterior primary divisions of the upper eleven thoracic 

 spinal nerves. The anterior primary division of the twelfth thoracic 

 spinal nerve is not an intercostal ner\'e, but lies along the lower 

 border of the twelfth rib, and is known as the subcostal nerve 

 (* last dorsal nerve '). 



The lower five intercostal nerves ultimately leave the intercostal 

 spaces and pass on to the anterior abdominal wall. 



Upper Six Intercostal Nerves. — Each of these nerves, as it enters 

 the back part of an intercostal space, lies between the posterior 



