This article is based on a study of the distribution of various types of nerve cells throughout the human central nervous system, a study which has been continuous over a period of twenty years. A consideration of minute histologic differences in cell type has been carefully avoided, and only those differences have been taken into account which can be clearly demonstrated and which are not materially affected by variations in technic or in the condition of the tissues. The function of nerve cells, unlike that of nerve fibers, cannot be studied directly, but one can study the differences between nerve cells employed in different types of nervous reactions, and can note the constant histologic features of nerve cells which always occur in various nervous mechanisms the activities of which exhibit in common some well marked feature. In making such a comparison between the character of reactions and the character of