Bones.The skeleton of the thorax is to a very considerable extent covered by muscles, so that in the strongly developed muscular subject it is for the most part concealed. In the emaciated subject, however, the ribs, especially in the lower and lateral regions, stand out as prominent ridges with the sunken intercostal spaces between them.
In the middle line, in front, the superficial surface of the sternum can be felt throughout its entire length at the bottom of a furrow, the sternal furrow, situated between the Pectorales majores. These muscles overlap the anterior surface somewhat, so that the whole width of the sternum is not subcutaneous, and this overlapping is greatest opposite the middle of the bone; the furrow, therefore, is wide at its upper and lower parts but narrow in the middle. At the upper border of the manubrium sterni is the jugular notch: the lateral parts of this notch are obscured by the tendinous origins of the Sternocleidomastoidei, which appear as oblique cords narrowing and deepening the notch. Lower down on the subcutaneous surface is a well-defined transverse ridge, the sternal angle; it denotes the junction of the manubrium and body. From the middle of the sternum the sternal furrow spreads out and ends at the junction of the body with the xiphoid process. Immediately below this is the infrasternal notch; between the sternal ends of the seventh costal cartilages, and below the notch, is a triangular depression, the epigastric fossa, in which the xiphoid process can be felt.
On either side of the sternum the costal cartilages and ribs on the front of the thorax are partly obscured by the Pectoralis major, through which, however, they can be felt as ridges with yielding intervals between them corresponding to the intercostal spaces. Of these spaces, that between the second and third ribs is the widest, the next two are somewhat narrower, and the remainder, with the exception of the last two, are comparatively narrow.
Below the lower border of the Pectoralis major on the front of the chest, the broad flat outlines of the ribs as they descend, and the more rounded outlines of the costal cartilages, are often visible. The lower boundary of the front of the thorax, which is most plainly seen by bending the body backward, is formed by the xiphoid process, the cartilages of the seventh, eighth, ninth, and tenth ribs, and the ends of the cartilages of the eleventh and twelfth ribs.
On either side of the thorax, from the axilla downward, the flattened external surfaces of the ribs may be defined. Although covered by muscles, all the ribs, with the exception of the first, can generally be followed without difficulty over the front and sides of the thorax. The first rib being almost completely covered by the clavicle can only be distinguished in a small portion of its extent.
At the back, the angles of the ribs lie on a slightly marked oblique line on either side of, and some distance from, the spinous processes of the vertebræ. The line diverges somewhat as it descends, and lateral to it is a broad convex surface caused by the projection of the ribs beyond their angles. Over this surface, except where covered by the scapula, the individual ribs can be distinguished.
Muscles.The surface muscles covering the thorax belong to the musculature of the upper extremity (Figs. 91215, 91219), and will be described in that section (page 1325). There is, however, an area of practical importance bounded by these muscles. It is limited above by the lower border of Trapezius, below by the upper border of Latissimus dorsi, and laterally by the vertebral border of the scapula; the floor is partly formed by Rhomboideus major. If the scapula be drawn forward by folding the arms across the chest, and the trunk bent forward, parts of the sixth and seventh ribs and the interspace between them become subcutaneous and available for ausculation. The space is therefore known as the triangle of ausculation.
Fig. 91215 The left side of the thorax.
Mamma.The size of the mamma is subject to great variations. In the adult nulliparous female, it extends vertically from the second to the sixth rib, and transversely from the side of the sternum to the midaxillary line. In the male and in the nulliparous female the mammary papilla is situated in the fourth interspace about 9 or 10 cm. from the middle line, or 2 cm. from the costochondral junction.