Scammon also proposed the difference in growth of different tissues and systems that was concluded from the analysis of the weights of the body and of the specific tissues and organs. According to him the different growth curves could be summarised into four patterns as depicted in the figure-3.3, that provide convenient means to understand the differential nature of postnatal growth. The data represented in the figure is relative and size attained by each type of tissue at each age is expressed as a percentage of the total increment between birth and 20 years of age (100%). The different patterns of growth among varied tissue and organs can be described as follows:
a) General Curve-The general or body curve describes the growth of the body as a whole and most of its parts- stature, weight, and external dimensions of the body. This curve is characteristic of most systems of the body, including muscle mass, the skeleton (except certain parts of skull and face), respiratory system, heart and blood vessels, digestive system, and the urinary system.
The growth pattern is generally S-shaped (sigmoid) and has four phases:
- rapid growth in infancy and early childhood,
- steady but rather constant growth during middle childhood,
- rapid growth during the adolescent spurt, and
- slow increase and eventual cessation of growth after adolescence .
- The latter part of the curve continues into the third decade of life for most dimensions .
b) Neural Curve –The neural curve characterizes the growth of the brain, nervous system, and associated structures, such as the eyes, upper face, and the parts of the skull. These tissues experience rapid growth early in postnatal life, so about 95% of the total increment in size of the central nervous system and related structures between birth and 20 years of age is already attained by about 7 years of age. These represent a steady gain after 7 years of age, with a slight growth spurt during adolescence. A small but definite spurt occurs in head length and breadth, but all or most of this is due to thickening of the skull bones and the scalp together with development of the air sinuses. The face follows a curve midway between that of the top portion of the skull and the remainder of the skeleton (www.universalium. academic.ru). It is nearer its mature dimensions at birth, but still has a considerable adolescent spurt, which is greatest in mandible. Thus the head as a whole is more advanced than the remainder of the body, and the top part of it i.e. the eyes and brain are more advanced than the lower portion that is, the face and the jaw.
c) Genital Curve –This curve characterises the growth pattern of the primary and secondary sex characteristics. Primary sex characters include the ovaries, fallopian tubes, uterus, and vagina in females; while the testes, seminal vesicles, prostate, and penis in males. Secondary sex characters involve the breasts in females, pubic and axillary hair in both exes, and facial hair and growth of larynx in males. Larynx growth is related with the voice changes that occur during male adolescence. The tissues show a slight growth in infancy, followed by a latent period during most of childhood. However, the rapid growth and maturation among these tissues is noticed during the adolescent spurt.
d) Lymphoid Curve-The curve represents the growth of lymph glands, thymus gland, tonsils, appendix, and the lymphoid patches of tissue in the intestine. These are in turn related with child’s development of immunological capacities; including the resistance to infectious diseases. They demonstrate a rapid growth during infancy and childhood, reaching a maximum when children are about 11-13 years of age. At these ages, the children posse s the lymphoid tissue twice the amounts they possess during their adulthood. The decline of lymphoid curve during the second decade of life is usually related to the involution (shrinking) of the thymus and tonsils at this time. The different types of Scammon’s curves indicate the differential nature of postnatal growth. Growth usually occurs in different areas and tissue of the body at different times and different rates. Although somewhat simplified and diagrammatic, the four curves give a sense of ord r to the structural and functional changes that occur with growth and maturation, however, with certain exceptions, like craniofacial skeleton. On one hand, the upper part of the face, the orbits of the eyes and the cranial vault follow the neural curve and complete a good portion of their growth when the child is about 7 year. 0 age. While on the other hand, the lower part of face, including jaw, follows the general curve and has an adolescent growth spurt. Thus! the upper part of the face has a different growth pattern than the lower part.