The nature of the transformations undergone by the skeleton of the trunk in relation to its different parts is substantially as follows: in the child at birth the vertebral column is straight, and the thorax is higher up than in the adult; the pelvis, on the contrary, slants forward and downward. In the adult the vertebral column is curved in the form of an S, showing the two-familiar dorsal-lumbar curves, and the axes of the thorax and pelvis are more perceptibly horizontal; in short, in the course of growth a descent of the thorax has taken place, together with a rotation of the pelvis (Fig. 124).
A. Descent of the Thorax.—This is the chief of these characteristics: the thorax descends in the course of its growth.
In the new-born child the upper edge of the manubrium of the sternum is in juxtaposition to the body of the first dorsal vertebra, while in the adult it is situated on a level with the lower edge of the second vertebra.
Even the tendinous arch of the diaphragm has shifted, being lowered by the space of a vertebra; it is situated between the eighth and ninth vertebræ in the child at birth, and between the ninth and tenth in the adult.
The outside characteristics are in correspondence with this fact; the shoulders descend in the course of growth. In the adult, the acromia or points of the shoulders are on a lower level than the incisura or cleft in the sternum (which is visible at the anterior base of the neck, and may be felt as an indented half-moon); while in the new-born child, on the contrary, the shoulders are higher up than the upper extremity of the sternum.
Another external characteristic of the descent of the thorax is the change in position of the nipples at successive ages; the mammary papillæ of the adult correspond to the level of the lower extremity of the sternum, and are situated respectively at the central points of the two halves of the thorax; in the new-born child, on the contrary, the mammary papillæ are further apart and higher up.
Fig. 125.—A = vertex of triangle; B B' = extremities of base, corresponding to the two nipples.
These characteristics of the descent of the thorax are fully established in the period of puberty and are of great importance, since, if not completed, they indicate cases of arrest of development or infantilism.
Quétélet has made a study of the triangulation of the thorax (Fig. 125).