OF THE THORAX 417 



rotate the vertebral column, so that the front of the trunk is turned to the side opposite to that 

 from which the muscle acts. 



Surface Forms. The surface forms produced^ by the muscles of the back are numerous and 

 difficult to analyze unless they are considered in systematic order. The most superficial layer, 

 consisting of large strata of muscle tissue, 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 of 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 thoracic 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 process of the twelfth thoracic 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 thorax. In this way it modulates the outline of the Erector spinae; 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 sur- 

 face 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 trans- 

 verse processes of the upper cervical vertebrae to the angle of the scapula, covered over and 

 toned down by the overlying Trapezius. The Rhomboidei produce, when in action, a vertical 

 eminence between the vertebral border of the scapula and the vertebral 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 

 not covered 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 Latis- 

 simus 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 spinae and its continua- 

 tions 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 vertebral 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 Sternomastoid and being 

 lost below beneath the Trapezius. 



Applied Anatomy. In cases of tuberculous caries of the vertebral bodies, and in other diseases 

 affecting the vertebral column, rigidity of the spinal muscles is one of the earliest and most 

 constant symptoms. A child with commencing vertebral disease always maintains the affected 

 portion of the column in a state of absolute rigidity, to prevent the inflamed structures from 

 being moved against each other; this is one of the best examples of nature's method of producing 

 rest of the affected part. 



II. MUSCLES AND FASCIJE 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. 



27 



