366 MUSCLES AND FASCIAE. 



by constricting the cavity of the abdomen, in which action they are materially 

 assisted by the descent of the Diaphragm. By these means the foetus is expelled 

 from the uterus, the feces from the rectum, the urine from the bladder, and ite 

 contents from the stomach in vomiting. 



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

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

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

 those of the two sides act alternately, rotation of the trunk at the same time 

 taking place to the opposite side. 



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

 as in climbing ; or, acting singly, they draw the pelvis upwards, and rotate the 

 vertebral column to one side or the other. 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 Pyra- 

 midales are tensors of the linea alba. 



The Quadratus Lumborum, by the portion inserted into the last rib, draws 

 down and fixes that bone, acting thereby as a muscle of forced expiration : by 

 the portion inserted into the lumbar vertebra?, it draws the spine towards the 

 ilium, and thus inclines the trunk towards its own side : or if the thorax and 

 spine be fixed, it may act upon the pelvis raising it towards its own side when 

 only one muscle is put in action, and when both muscles act together, either 

 from below or above, they flex the trunk. 



MUSCLES OF THE THORAX. 



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

 number. They are the 



Intercostales Externi. Infracostales. 



Iritercostales 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 mus- 

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

 muscular fibres. These are the intercostal fasciae ; 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 Intercos- 

 tals and spine, behind. 



The Intercostal Muscles (Fig. 237) are two thin planes of muscular and ten- 

 dinous structure, 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 name "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. B} 7 their outer surface, with the muscles which immediately invest 

 the chest, viz., the Pectoralis Major and Pectoralis Minor, Serratus Magnus, 

 Ehomboideus Major, Serratus Posticus Superior and Inferior, Scalenus Posticus, 

 Sacro-lumbalis, Longissimus Dorsi, Cervicalis Ascendens, Transversalis Colli, 

 Levatores Costarum, and the Obliquus Externus Abdominis. By their internal 

 surface, with a thin layer of fascia, which separates them from the intercostal 



