This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The proportions of the body of the new born infant are very different from those of the adult. The umbilicus is about the middle, the lower limbs and the chest are small, and the head, as compared with the rest of the body, is much larger than afterwards. In representing the adult figure, the rule recognised in art is to allow eight timeS the perpendicular length of the head for the height of the whole body, thus: from the crown to the chin, from the chin to the level of the nipples, from that level to the navel, and from the navel to the pubis, each one head; and from the pubis to the lower part of the knee, and thence to the sole, each two heads. The position of the umbilicus is, as will be perceived from this, considerably higher in the adult than the child.
The pelvis in the child is remarkably small, and is so situated at birth that the sacrum lies pretty nearly in a continuation of the line of the vertebral column; but when the child begins to walk, the stretching out of the thighs is effected much more by bending the sacrum back than by movement at the hip joints, and the brim of the true pelvis is thrown into a nearly vertical plane, whereas in the adult it is at an angle of about 30° with the vertical plane. This circumstance, in conjunction with the shortness of the chest, is the cause of the prominent appearance of the abdomen in children.
The growth of the head deserves especial attention. The maximum proportion of the cranium to the rest of the body is found in the early state of the embryo, and it continues to diminish till maturity is reached. At birth, the parietal region is particularly prominent, while both frontal and occipital regions are comparatively small. During childhood, the forehead becomes sufficiently developed to produce that projection forwards at the level of the frontal eminences, which gives a characteristic appearance to the child's head; but these eminences continue to ascend to a higher level above the eyes, and to separate one from the other for a number of years; and the projection forwards of the lower part of the forehead, including the frontal sinus, is not completed till after puberty. A comparatively small development both of the frontal and the occipital region is characteristic of the female sex.
The face is exceedingly small in children.' It undergoes development in connection with both first and second dentition; but it does not reach its full proportions till after puberty, and remains permanently smaller in females than in males.
The eyeballs and the internal and middle ear are nearly as large at birth as in the adult.
Probably the period of maximum vital energy is best indicated by the vital capacity of the chest, which is greatest about the age of thirty. As age advances, the diminished rapidity of the pulse, and greater difficulty in repair of injuries, indicate the decline of nutritive activity, and at last, as years accumulate, this becomes insufficient to support the processes necessary for life.