NORMAL HISTOLOGY. 



the course of the nerve-trunks, and are found within the medulla in 

 considerable numbers. 



The lymphatics are represented by delicate canals within the 

 fibrous septa which communicate with the intercellular clefts of both 

 cortex and medulla on the one hand, and with the larger lymph-ves- 

 sels within the capsule on the other. 



THE DEVELOPMENT OF THE NERVOUS TISSUES. 



The consideration of the general changes involving the primary 

 neural tube and its cephalic expansions, the brain-vesicles, by which 

 are produced the various portions of the cerebro-spinal axis, belongs 

 to embryology, and lies without our present purpose ; an account 

 of the histogenesis of the nervous tissues, however, is of much 

 interest in indicating the true relations of the structural components 

 of the great nervous masses. 



The essential parts of the nervous system, including the nerve- 

 cells, the nerve-fibres, and the neuroglia, are developed from the 

 ectoderm alone, and all result from the differentiation and speciali- 

 zation of the walls of the primary neural tube. 



This canal is formed by the gradual closure of the early furrow, 



the medullary groove, of the 



FIG. 356. invaginated ectoderm along 



the dorsal line ; by the approxi- 

 mation of the upper or dorsal 

 edges of the involution the 

 furrow is converted into a 

 tube, the sides, or medullary 

 plates, of the extreme ante- 

 rior and posterior segments of 

 which are the last to unite. 

 Even before the closure of the 

 groove has been completed a 

 differentiation of two impor- 

 tant regions is indicated; these 

 are the areas giving rise to the segmental ganglia and to the 

 general axial nerves. 



The area for the latter is represented by the lining of the neural 

 tube, that for the former by the inconspicuous cell-groups lying on 

 either side of the line of closure. 



The primary wall of the neural tube consists of a single layer of 

 columnar epithelial cells, whose nuclei occupy the middle third of 

 the elements, leaving an outer and an inner free zone ; within the latter 

 appears very early a second variety of cell, which is distinguished 

 by its large spherical form and conspicuous nucleus. The 



Section of nine-day rabbit embryo, showing open 

 neural tube : e, ectoderm invaginated and thickened 

 within neural canal () ; m, mesoderm ; b, body- 

 cavity ; g. still open gut, lined by entoderm (h). 



