The establishment of a biologic timescale of human development requires observation 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 over 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 about 24 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 knowledge of normal development and disease around the timetables for human growth and development listed in Tables 27-1 and 27-2. In addition, the last several decades have brought major advances in the understanding of the genetic and molecular control of neural development. That topic is considered in Chap. 37.
Neuroanatomic Bases of Normal Development
A large body of knowledge has accumulated concerning the functional and structural status of the nervous system during each successive period of life. This information is reviewed briefly in the following paragraphs and is summarized in Table 27-2. It must be kept in mind that development of the nervous system does not proceed stepwise, from one period to the next, but is continuous from conception to maturity. The sequences of development are much the same in all infants, although the rate may vary slightly. Any given behavioral function, in order to be expressed, must await the development of its neural substrate. Furthermore, at any given moment in development, several measurable functions appear in parallel, and it is often a dissociation between them that acquires clinical significance.