If the two nipples and the sternal incisura are connected by straight lines inclosing an isosceles triangle ABB´, the length of the base in the new-born child is 70 millimetres, and that of the sides BA, B´A is 54 millimetres, and the height 41 millimetres.
In the adult the dimensions are as follows: BB´ = 197 millimetres; AB, AB´= 184 millimetres; and the height = 155 millimetres. Comparing the measurements of the child at birth with those of the adult, we find that the base in the adult is 2.81 times, and the side 3.41 times that of the child; in other words, the sides of the triangle increase far more than the base, and its height in the adult (representing very nearly the entire height of the sternum), is 3.78 times that in the new-born child. Consequently, in the course of its transformation the thorax not only descends, but it is also lengthened in the adult, as compared with the form that it had at birth.
B. Dimensions of Thorax in Relation to Stature.—Besides its descent, there is a second transformation of the thorax, in regard to its volumetric relations to the rest of the body. The perimeter of the thorax and the circumference of the head are pretty nearly equal in the new-born child; if anything, the circumference of the thorax is a trifle less than that of the head; but when it equals it, this is a sign of robustness. In the majority of cases it is not until the second year or thereabouts that the two circumferences become equal. If, however, such inequality should still persist after the child had entered upon the third year, it would constitute a sign of rickets (head too large, chest too narrow).
As to the relations between the thoracic circumference and the stature, it is found that in the child at birth the thoracic circumference exceeds one-half the stature by about 10 centimetres. If the difference is less than 8 centimetres it is a sign of feeble constitution, if it is greater than 10 (for instance, 11 centimetres) it is a sign of great robustness.
This difference disappears little by little; at the age of five years it is already reduced to between 4 and 5 centimetres; at the age of fifteen, the period of puberty, it has wholly disappeared, and the well-known relation between the stature and the circumference of the thorax has become established; the thoracic circumference is equal to one-half the stature (see chapter on Form), and this constitutes Goldstein's vital index:
Vi = (100×Tc)/(S)
As early as 1895, Pagliani published some studies of children, which reveal the physiological importance of the dimensions of the thorax; watching the lives of infants whose measurements he took at the foundling asylum, he observed that the mortality of infants is quite rare when they exceed the above proportions between circumference of chest, head, and stature.
From a study of 452 infants, Fraebelius has drawn the following conclusions:
I. Mortality 21 per cent.; circumference of thorax greater than half the stature by 9.10 centimetres; circumference of thorax less by 1.5 centimetres than perimeter of cranium.
II. Mortality 42.9 per cent.; circumference of thorax greater by 7 centimetres than one-half the stature; circumference of thorax less by 2.8 centimetres than circumference of cranium.