THE THORACIC MUSCLES. 



539 



2. Intercostales Interni (Fig. 537). 



Attachments. — The internal intercostals lie immediately beneath the external 

 and, like these, extend across each of the intercostal spaces. The fibres have a 

 direction almost at right angles to those of the external intercostals, being directed 

 obliquely downward and inward from the lower border of one rib and its costal car- 

 tilage to the upper border of the next. The muscle-sheets s'o formed extend from 

 the medial extremity of each intercostal space as far back as the angles of the ribs, 

 becoming there continuous with an internal intercostal fascia or posterior intercostal 



Fig. 537. 



I rib Clavicle 



External intercostal fascia 



Internal intercostal mus- 

 cles, anterior part covered 

 bv e-xternal fascia 



Internal intercostal muscles, 

 exposed after removal of external 



I'pper external intercostal 



muscles 



Internal intercostal fascia 



Lower external intercostal 

 muscles * 



Dissection of thoracic wall of left side, showing intercostal muscles and fasciae. 



aponeurosis which continues backward to the tubercles of the ribs. The medial fibres 

 of the muscles of the lower two intercostal spaces become continuous with the upper 

 portion of the internal oblique muscle of the abdomen. 



Nerve-Supply. — The anterior divisions (intercostal nerves) of the thoracic 

 nerves. 



Action. — To draw the ribs upward. 



The Subcostal Muscles. — Posteriorly the fibres of the various internal intercostals do not 

 confine themselves to a single intercostal space, but extend downward to the next space below, 

 spreading out in the muscle-sheet of that space. These fibres, which vary greatly in the ex- 

 tent of their development, form what are termed the subcostal muscles. 



