OF THE THORAX. 441 



consisting of large strata of muscular substance, influences to a certain extent the surface form, 

 and at the same time reveals the forms of the layers beneath. The Trapezius at the upper part 

 of the back, and in the neck, covers over and softens down the outline of the underlying muscles. 

 Its anterior border forms the posterior boundary of the posterior triangle of the neck. It forms 

 a slight undulating ridge which passes downward and forward from the occiput to the junction 

 of the middle and outer third <jf the clavicle. The tendinous ellipse formed by a part of the 

 origin of the two muscles at the back of the neck is always to be seen as an oval depression, 

 more marked when the muscle is in action. A slight dimple on the skin opposite the interval 

 between the spinous processes of the third and fourth dorsal vertebrae marks the triangular 

 aponeurosis by which the inferior fibres are inserted into the root of the spine of the scapula. 

 From this point the inferior border of the muscle may be traced as an undulating ridge to the 

 spinous pro -ess of the twelfth dorsal vertebra. In like manner, the Latissimus dorsi softens 

 down and modulates the underlying structures at the lower part of the back and lower part of 

 the side of the chest. In this way it modulates the outline of the Erector spinse ; of the Serratus 

 posticus inferior, which is sometimes to be discerned through it, and is sometimes entirely 

 obscured by it : of part of the Serratus magnus and Superior oblique, which it covers ; and of 

 the convex oblique ridges formed by the ribs with the intervening intercostal spaces. The 

 anterior border of the muscle is the only part which gives a distinct surface form. This border 

 may be traced, when the muscle is in action, as a rounded edge, starting from the crest of the 

 ilium, and passing obliquely forward and upward to the posterior border of the axilla, where it 

 combines with the Teres major in forming a thick rounded fold, the posterior boundary of the 

 axillary space. The muscles in the second layer influence to a very considerable extent the surface 

 form of the back of the neck and upper part of the trunk. The Levator anguli scapulae reveals 

 itself as a prominent divergent line, running downward and outward, from the transverse pro- 

 cesses of the upper cervical vertebrae to the angle of the scapula, covered over and toned down 

 by the overlying Trapezius. The RkamboAIa, produce, when in action, a vertical eminence 

 between the internal border of the scapula and the spinal furrow, varying in intensity according 

 to the condition of contraction or relaxation of the Trapezius muscle, by which they are for the 

 most part covered. The lowermost part of the Rhomboideus major is uncovered by the Trapezius, 

 and forms on the surface an oblique ridge running upward and inward from the inferior angle 

 of the scapula. Of the muscles of the third layer of the back, the Serratus posticus superior 

 does not in any way influence surface form. The Serratus posticus inferior, when in strong 

 action, may occasionally be revealed as an elevation beneath the Latissimus dorsi. The Splenii 

 by their divergence serve to broaden out the upper part of the back of the neck and produce a 

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

 muscles those of the fourth layer the Erector *r>!i< ff 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 TracheJo-mastofd, 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. 



THE THORAX. 



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

 number. They are the 



\ Intercostales externi. Infracostales. 



Intercostales interni. Triaugularis sterni. 



Levatores costarum. 



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 archest 

 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. 299) 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 i- 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. 



