TBE MUSCLES. 237 



3. External Intercostals. (Fig. 104, 68.) 

 Situation — Form. — These muscles fill the spaces between the ribs, but 

 do not descend beyond their inferior extremities ; they, therefore, do not 

 occupy the intervals between the cartilages. They are flattened, fleshy 

 bands, gradually diminishing in thickness from above to below. 



Structure — Attachments. — Each intercostal muscle is composed of a 

 series of muscular fasciculi, intermixed with numerous aponeurotic fibres, 

 both of which pass obliquely backwards and downwards, from the posterior 

 border of the preceding to the external face of the succeeding rib. 



Belations. — They respond, outwardly, to the differeni muscles applied 

 against the thoracic walls ; inwardly, to the internal intercostals. 



4. Internal Intercostals. (Kg. 106, 16.) 



These are placed at the internal face of the preceding, which they exactly 

 repeat with regard to their general form, but from which they difier in the 

 following points : 



1. Very thick between the costal cartilages, these muscles are reduced 

 at the upper part of the intercostal spaces to a thin aponeurotic layer, 

 supported only by some fleshy fibres. They therefore 'gradually diminish 

 in thickness from below upwards. 



2. Their fasciculi are less tendinous than those of the external inter- 

 costals, and are carried obliquely forward and downward, from the anterior 

 border of the posterior rib to the posterior border and internal face of the 

 rib in front ; so that the fibres of the external and internal intercostals*- cross 

 each other like the letter X. 



3. Outwardly, they respond to the external intercostals ; inwardly, to 

 the costal pleura. 



Action of the intercostal muscles. — The function of these muscles has been 

 for a long time, and is even now, much discussed ; and it may be said that 

 there were never, perhaps, more diverse or contrary opinions given on any 

 subject than on this. M. Berard, who has summed up the elements of the 

 discussion with the greatest judgment, considers the external intercostals as 

 inspiratory muscles, and the internal ones also as inspiratory by those 

 fasciculi which occupy the spaces between the costal cartilages ; the 

 remainder, the majority, are expiratory. 



5. Supercostals. 

 Synonyms. — Transverso-costales — Girard. (Levatores costarum — Perdvall.') 

 Small, flat, triangular, muscular and tendinous fasciculi, constituting, 

 it might be said, the heads of the external intercostals, from which they are 

 scarcely distinguishable in the first and last costal intervals. 



They arise from the transverse processes of the dorsal vertebra, and are 

 directed backwards and outwards, gradually expanding, to terminate on the 

 external face of the one or two ribs which succeed their fixed insertion. 

 Outwardly, they respond to the Uio-spinalis ; inwardly, to the external 

 intercostals. 



The supercostals draw the ribs forward, and are consequently inspiratory 

 muscles. 



6. Triangularis of the Sternum. 



Synonyms. — Sternalis — Bonrqelat. Sterno-costalis — Girard. 'The sterno-costales of 

 Percivall, and sterno-costalis of Leyh.) 



