LEVATORES COSTARUM 397 



eciluinn. It \vill 1)0 also ni)ticed that the front })art of tlie external, and the hack i)art 

 of the internal intercostal sheets are thin and weak, so as to diniinisli this tendency 

 towards an ex])iratorv movement. It has been urged that both sets of intercostals 

 cannot approximate the ril)s, as in inspiration some of the intercostal spaces are seen 

 to open out and become wider. To this it may be replied that, whatever happens in 

 some of the spaces, it is certain that the general tendency is that of approximation 

 of the ribs, and diminution of the intervals between them, as after a full inspiration 

 the last rib is nearer to the first rib. Moreover, it does not always follow that 

 because a muscle is lengthening it is not acting (cf. such cases as that of the long 

 head of the triceps when it is used in extending the elbow during the elevation 

 of the arm)- If when most of the ribs are being approximated it shovdd happen 

 that some of tlie spaces are found to be widened, the action of the muscle in these 

 widened spaces will probal^ly be to prevent a greater separation, and by steadying 

 the lower ribs to enable the muscles which descend from them to act. When the 

 lowest rib is fixed by the quadratus lumborum and other muscles, it is probal)le 

 that the intercostals, at any rate those of the lower spaces, by approximating 

 the lower ribs to the last rib, may act as muscles of expiration. The follow- 

 ing are some of the arguments which may be adduced in support of the view 

 that both sets of intercostals act together, and also that their action is usually 

 inspiratory. 



1. The advantage already mentioned, from their o1:)lique decussation enabling 

 them to approximate the ribs more completely and at the same time to elevate them 

 directly when they act in combination (cf. the action of the external and internal 

 ol)lique in approximating the last rib to the crest of the ilium). 



2. ^luscles supplied by the same nerve are rarely antagonistic. 



3. In long-standing paralysis of the intercostals the sternum is depressed, the 

 chest flattened, and kept in a permanent condition of exaggerated expiration. 



4. Galvanism of the intercostals produces expansion of the chest. In Du- 

 chenne's experiment he found that when he galvanised so slightly as only to affect 

 the external intercostals, the inspiratory movement was small; but when he gal- 

 vanised so strongly as to affect the nerve, and through the nerve the internal 

 intercostals — as was known by the contraction of muscular fibre being felt between 

 the cartilages of the ribs, in which situation the only fleshy fibres are those of the 

 inner set — a strong inspiratory movement was produced. 



From the peculiar shape of the ribs and the mode of their articulation, their 

 elevation is accompanied b}'' a rotation of the arcs formed by them upon their 

 chords so that their planes from a slojjing attain an almost horizontal position. 

 Hence the widening of the chest during inspiration, in addition to its expansion 

 from before backwards due to the forward elevation of the tips of the ribs. 



3. LEVATORES COSTARUM 



The levatores costarum (fig. 299) — named from their action, as elevators 

 of the ribs — are twelve triangular sheets, winch cover the back part of the inter- 

 costal spaces, and are continuous with the fibres of the external intercostal 

 muscles. 



Origin. — The tips of the transverse processes of the last cervical, and all the 

 thoracic vcrteln'ic except the last. 



Insertion. — The outer surface of the ribs from the tubercle to the angle. 



Structure. — Arising by short tendinous fibres the nuiscle expands in a fan 

 sha}K', and is attached to the next i-il^ lielow. Fre(iuently fil)res pass over one rib 

 and are inserted upon the next but one. 



Nerve-supply. — The intercostal nerves which send l)ranches to their deep 

 surfaces. 



Action. — To elevate the ribs in inspiration. 



Relations. — Superficially, the outer and upward continuations of the erector 

 spina-; deeply, the external intercostals, which are continuous with the outer border 

 of the muscles. 



