350 THE MUSCLES AND FASCIAE 



local fulness in this situation, but do not otherwise influence surface form. Beneath all these 

 muscles those of the fourth layer the Erector spince and its continuations influence the surface 

 form in a decided manner. In the loins, the Erector spinae, bound down by the lumbar fascia, 

 forms a rounded vertical eminence, which determines the depth of the spinal furrow, and which 

 below tapers to a point on the posterior surface of the sacrum and becomes lost there. In the 

 back it forms a flattened plane which gradually becomes lost. In the neck the only part of this 

 group of muscles which influences surface form is the Trachelo-mastoid, which produces a short 

 convergent line across the upper part of the posterior triangle of the neck, appearing from 

 under cover of the posterior border of the Sterno-mastoid and being lost below beneath the 

 Trapezius. 



2. MUSCLES AND FASCLE OF THE THORAX. 



The muscles belonging exclusively to this region are few in number. They are the 



Intercostales externi. Triangularis sterni. 



Intercostales interni. Levatores costarum. 



Infracostales. Diaphragm. 



Intercostal Fasciae. 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, the middle intercostal fascia, 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 

 intercostals and spine, behind. 



The Intercostal muscles (Fig. 230) are two thin planes of muscular and tendinous 

 fibres, 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 

 tendinous fibres are longer and more numerous than the muscular ; hence the walls 

 of the intercostal spaces possess very considerable strength, to which the crossing of 

 the muscular fibres materially contributes. 



The External Intercostals are eleven in number on each side. They extend 

 from the tubercles of the ribs, behind, to the commencement of the cartilages of 

 the ribs, in front, where they terminate in a thin membrane, the anterior inter- 

 costal membrane, which is continued forward to the sternum. They arise from 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 cartilages, and in the upper 

 two or three spaces they do not quite extend to the ends of the ribs. Their fibres 

 are directed obliquely downward and forward, in a similar direction with those of 

 the External oblique muscle of the abdomen. They are thicker than the Internal 

 intercostals. 



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

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

 major, Serratus posticus superior and inferior, Scalenus posticus, Ilio-costalis, 

 Longissimus dorsi, Cervicalis ascendens, Transversalis cervicis, Levatores costarum, 

 Obliquus externus abdominis, and the Latissimus dorsi ; by their internal surface, 

 with the middle intercostal fascia, which separates them from the intercostal vessels 

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



The Internal intercostals are also eleven in number on each side. They com- 

 mence anteriorly at the sternum, in the interspaces between the cartilages of the 

 true ribs, and from the anterior extremities f the cartilages of the false ribs, and 

 extend backward as far as the angles of the ribs, whence they are continued to the 

 vertebral column by a thin aponeurosis, the posterior intercostal membrane. They 

 arise from the ridge on the inner surface of each rib, as well as from the corre- 

 sponding costal cartilage, and are inserted into the upper border of the rib below. 

 Their fibres are directed obliquely downward and backward, passing in the oppo- 

 site direction to the fibres of the External intercostal muscle. 



Relations. By their external surface, with the intercostal vessels and nerves 

 and the External intercostal muscles ; near the sternum, with the anterior inter- 



