HISTOLOGICAL ELEMENTS 



857 



blasts that migrate, in the course of development, from the rudiments 

 of the spinal ganglia, and gathering in clumps form the ganglia of the 

 sympathetic chain (His). They agree in general with the cells of the 

 cerebro-spinal axis in possessing an axon and one or more, commonly 

 several, dendrites, although a few of them are devoid of dendrites. 

 The great majority of the axons remain unmedullated, but a few 

 acquire a very fine medullary sheath. 



The epithelium lining the central canal of the cord and the ventricles 

 of the brain has also been considered by some as of nervous nature. 

 The fact that the deep ends of the cells are continued into processes 

 which pierce far into the grey substance has been supposed to lend 

 weight to this opinion, but there is no good ground for it. 



Growth of Neurons. The growth of a neuron from origin to com- 

 pletion is a comparatively slow process in the higher animals. Early 

 in foetal life (about the third or fourth week in man) certain round 

 germinal cells make their appearance amid the columnar ectodermic 

 cells surrounding the neural canal. From their division are formed, 

 in the first months of embryonic life, the primitive nerve-cells or 

 neuroblasts. These soon elongate and push out processes, first the 



Fig. 33,7. Isolated nerve-cells from the spinal cord of a tadpole growing in clotted 

 Ivmph. A, B, C, are cells in different stages of growth. The lower view of C 

 was drawn under the microscope 4$ hours later than the upper (Harrison). 



axon or axons, and then the dendrites (Fig. 333). The formation of 

 the axons from the nerve-cell is most clearly followed in isolated 

 cultures (Fig. 337). As development goes on, the cell-body grows 

 larger, and the processes longer and more richly branched. The axon 

 and its collaterals, when it has any, in the case of the great majority of 

 the nervous elements of the brain and cord, ultimately acquire a 

 medullary sheath, although, as we have said, the time at which medul- 

 lation is completed varies in different groups of elements, and in some 

 nervous tracts it is even wanting at birth. At birth, too, the branches 

 of many of the cells are less numerous, and the connections between 

 different nervous elements therefore less intimate than they will after- 

 wards become. For many years the processes, and particularly the 

 axons, continue not only to grow longer, but to grow thicker as well. 

 The cell-body also enlarges, and the quantity of material in it that 

 stains with basic dyes increases. In the growing (lumbar) spinal 

 ganglia of the white rat the increase in volume of the largest cell- 

 bodies is very closely correlated with the increase in area of the cross- 

 section of the nerve-fibres growing out of them. The cross-section 

 of the axis-cylinder is, and remains, almost exactly equal to the area 



