b. The more essential parts vary less than the accessory parts in the course of their transformations.
The Index.—The index is the mathematical relation between the measurements belonging to the same individual, and as such it gives us an idea of the form; since the form is determined by the relations between the various parts constituting the whole.
The Stature
While the figure and the type of stature tend to delineate the individual considered by himself, the different measurements considered separately may guide us in our study of individuals in their relation to the race and the environment.
Among the measurements of the form, we will limit ourselves to a study of the stature and the weight, which serve to give us respectively the linear index of development and the volumetric estimate of the body taken as a whole. We shall reserve the study of the other measurements, such as the total spread of the arms and the perimeter of the thorax, until we come to the analytical investigation of the separate parts of the body (limbs, thorax).
The stature is expressed by a linear measure determined by the distance intervening in a vertical direction between the plane on which the individual is standing in an erect position and the top of his head.
It follows that the stature is a measurement determined by the erect position; on the other hand, when a man is in a recumbent position, what we could determine would be the length of body, which is not identical with the stature.
In fact, a man on foot, resting his weight upon articulations that are elastic, and therefore compressible, is a little shorter than when he is recumbent.
If we examine the skeleton (see Fig. 9), we discover that the single synthetic measure that constitutes the stature results from a sum of parts that differ greatly from one another. To be specific, it is composed of the long and short bones of the lower limbs; of flat bones, such as the pelvis and the skull; of little spongy bones, such as the vertebræ; all of which bones and parts obey different laws in the course of their growth. Furthermore, intervening between these various bones are soft, elastic parts, known as the articulations, which, starting from below, succeed each other in the following order:
- Calcaneo-astragaloid, between the calcaneus and the superimposed astragalus.
- Tibio-astragaloid, between the astragalus and the superimposed tibia.
- Of the knee, between the tibia and the femur.
- Of the hip, between the femur and the os innominatum.
- Sacro-iliac, between the os iliacum and the sacrum.
- Sacro-vertebral, between the sacrum and the last lumbar vertebra.
- Of the vertebræ, consisting of 23 intervertebral disks, that is to say interposed between the vertebræ, which include the following: 5 lumbar, 12 thoracic, 7 cervical.
- Occipito-atloid, between the first cervical vertebra, called the atlas and the os occipitale of the cranium.