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In this chapter and the next one on aging, we consider the effects of growth, maturation, and aging on the nervous system. These are discussed in some detail because certain aspects of neurologic diseases are meaningful only when viewed against the background of these natural age-linked changes. Developmental diseases of the nervous system, e.g., malformations and other forms of damage that are acquired in the intrauterine period of life, are considered in Chap. 38.

The establishment of a biologic timescale of human development requires observation, under standardized conditions, of a large number of normal individuals of known ages and testing them for measurable items of behavior. Because of individual variations in the tempo of development, it is equally important to study the growth and development of any one individual for a prolonged period. If these observations are to be correlated with stages of neuroanatomic development, the clinical and morphologic data must be expressed in units that are comparable. Early in life, very precise age periods are difficult to ascertain because of the difficulty in fixing the time of conception. The average human gestational period is 40 weeks (280 days), but birth may occur with survival as early as 28 or as late as 49 weeks (a time span of almost 5 months), and the extent of nervous system development varies accordingly.

After birth, any given item of behavior or structural differentiation must always have two reference points: (1) to a particular item of behavior that has already been achieved, and (2) to units of chronologic time or duration of life of the organism. The chronologic or biologic scale assumes special significance in early prenatal life. During that period development proceeds at such a rapid pace that small units of time weigh heavily and the organism appears to change literally day by day. In infancy, the tempo of development slows somewhat but is still very rapid in comparison with later childhood.

The neurologist will find it advantageous to organize his knowledge of normal development and disease around the timetables for human growth and development listed in Tables 28-1 and 28-2. In addition, the last decade has brought startling advances in the understanding of the genetic and molecular control of neural development. That topic is beyond the scope of this chapter, although the important subject of the mutations that are now known to be relevant to the developmental diseases of the nervous system are considered in Chap. 38.

Table 28-1 Timescale of Stages in Human Growth and Development

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