III. Mortality 67.5 per cent.; circumference of thorax greater by 4.5 centimetres than one-half the stature; circumference of thorax less by 4.7 centimetres than the cranial circumference.
The thorax in children of five years and upward ought to be larger by a few centimetres (not more than from 4 to 5) than one-half the stature.
C. Transformations of the Thorax Considered by Itself: Alterations in Shape.
Thoracic Index.—Lastly, the thorax changes its shape in the course of growth. In the new-born child it is very prominent in front, and narrow laterally; in the adult, on the contrary, it is more flattened in its antero-posterior dimension and wider transversely. Consequently the transformation consists in a notable difference in the proportion between the width and depth of the chest, that is, between the antero-posterior and the transverse diameters (see chapter on Technique). This proportion constitutes the thoracic index, which is expressed by the following formula:
Ti = (100A-PD)/TD
and this formula gives an idea of the shape of the thorax.
In the child at birth the antero-posterior diameter is very nearly equal to the transverse; accordingly, the index, at birth, oscillates between 90 and 100.
In the adult, however, the thoracic index is on an average 75; the transverse diameter therefore increases much more than the antero-posterior diameter. According to Quétélet, while the transverse diameter multiplies threefold in the course of its growth, the antero-posterior merely doubles (2.36); in addition to this the thorax also lengthens, as we have already seen.
Proportion, Shape and Dimensions of the Thorax.—In the adult normal man we find the following proportions: The distance between the mammary papillæ is about equal to the antero-posterior diameter of the thorax (hence the papillæ indicate the depth of chest) and is also perceptibly equal to one-half the breadth of the shoulders (measured between the two acromia), which, by the way, is the maximum transverse dimension of the skeleton.