Changes due to age in internodal length in the sural nerve in man.

In the peripheral nerve trunks of vertebrates, the distance between the nodes of Ranvier is normally greater on fibres of larger diameter than on those of smaller size (Ranvier, 1875; Key and Retzius, 1876). At the time of myelination during development , internodal length is uniformly short, the nodes being spaced about 150 to 300 ,u apart. Each inter-node represents the territory of an individual Schwann cell and it is of interest that the basic length of Schwann cells in tissue culture is of this order (Weiss and Wang, 1945). As growth proceeds, the number of nodes normally remains constant and the internodes therefore increase in length (Boycott, 1904). It has been shown that there is a direct pro-portionality between internodal length and the growth in length of the part in which the nerve lies (Vizoso, 1950; Thomas, 1955). All the fibres in a peripheral nerve trunk do not become myelinated at the same time and those that become myelinated later remain of smaller diameter (Thomas, 1956). As these fibres will be stretched less during growth, their internodal length will be shorter The dependence of internodal length on growth is also shown by the observation that in nerve fibres that have regenerated after Wallerian degeneration in an adult animal when growth has ceased, intemodal length is short on fibres of all diameters During development, the growth rates of inter-nodal length and fibre diameter differ and the precise relationship between these two variables in a nerve is therefore complex and alters during development (see Thomas, 1955). In adult mammalian nerves, including man, it is approximately linear