288 MUSCLES AND FASCIAE. 



2. If the spine is fixed, these muscles can compress the lower part of the thorax, 

 materially assisting in expiration. If the spine is not fixed, the thorax is bent 

 directly forward, if the muscles of both sides act, or to either side if they act 

 alternately, rotation of the trunk at the same time taking place to the opposite 

 side. 



3. If the thorax is fixed, these muscles, acting together, draw the pelvis upwards, 

 as in climbing ; or, acting singly, the pelvis is drawn upwards, and the vertebral 

 column rotated to one or the other side. The Eecti muscles, acting from below, 

 depress the thorax,. and consequently flex the vertebral column; when acting from 

 above, they flex the pelvis upon the vertebral column. The Pyramidales are 

 tensors of the linea alba. 



MUSCLES AND FASCIJS OF THE THOEAX. 



The muscles exclusively connected with the bones in this region are few in 

 number. They are the 



Intercostales Externi. Infra-costales. 



Intercostales Interni. Triangularis Sterni. 



Levatores Costarum. 



Intercostal Fascise. A thin but firm layer of fascia covers the outer surface of 

 the External intercostal and the inner surface of the Internal intercostal muscles ; 

 and a third layer, more delicate, is interposed between the two planes of muscular 

 fibres. These are the intercostal fascias ; they are best marked in those 'situations 

 where the muscular fibres are deficient, as between the External intercostal 

 muscles and sternum, in front ; and between the Internal intercostals and spine, 

 behind. 



The Intercostal Muscles are two thin planes of muscular and tendinous struc- 

 ture, placed one over the other, filling up the intercostal spaces, and being directed 

 obliquely between the margins of the adjacent ribs. They have received the names 

 " external" and " internal," from the position they bear to one another. 



The External Intercostals are eleven in number on each side, being attached 

 to the adjacent margins of each pair of ribs, and extending from the tubercles of 

 the ribs, behind, to the commencement of the cartilages of the ribs, in front, 

 where they terminate in a thin membranous aponeurosis, which is continued for- 

 wards to the- sternum. They arise from the outer lip of the groove on the lower 

 border of each rib, and are inserted into the upper border of the rib below. 

 In the two lowest spaces they extend to the end of the ribs. Their fibres are 

 directed obliquely downwards and forwards, in a similar direction with those of 

 the External oblique muscle. They are thicker than the Internal intercostals". 



Relations. By their outer surface, with the muscles which immediately invest 

 the chest, viz., the Pectoralis major and minor, Serratus magnus, Rhomboideus 

 major, Serratus posticus superior and inferior, Scalenus posticus, Sacro-lumbalis 

 and Longissimus dorsi, Cervicalis ascendens, Transversalis colli, Levatores costa- 

 rum, and the Obliquus externus abdominis. By their internal surface, with a thin 

 layer of fascia, which separates them from the intercostal vessels and nerve, the 

 Internal intercostal muscles, and, behind, from the pleura. 



The Internal Intercostals, also eleven in number on each side, are placed on 

 the inner surface of the preceding, commencing anteriorly at the sternum, in the 

 interspaces between the cartilages of the true ribs, and from the anterior extremi- 

 ties of the cartilages of the false ribs; and extend backwards as far as the angles 

 of the ribs, where they are continued to the vertebral column by a thin aponeu- 

 rosis. They arise from the inner lip of the groove on the lower border of each 

 rib, as well as from the corresponding costal cartilage, and are inserted into the 

 upper border of the rib below. Their fibres are directed obliquely downwards 

 and backwards, decussating with the fibres of the preceding. 



Relations. By their external surface, with the External intercostals, and the 



