THE CENTRAL NERVOUS SYSTEM 



755 



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 medullation is com- 

 pleted 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 

 afterwards become. For many years the processes, and particu- 

 larly the axons, continue not only to grow longer, but also to 

 grow thicker. The cell-body also enlarges, and the quantity of 

 material in it that stains with basic dyes increases. In the 



2. 



FIG. 309. SECTION 

 THROUGH HALF OF 

 NEURAL TUBE (BAR- 

 KER, AFTER HlS). 



The pear-shaped neuro- 

 blasts are seen migrating 

 outwards. The axons of 

 some of them are seen 

 pushing their way out 

 through the marginal veil 

 as the anterior root of a 

 spinal nerve. 



J. 



FIG. 310. 



i, spinal ganglion cells of a still-born male child ; 



2, of a man ninety-two years old ( x 250) N, nuclei ; 



3, nerve-cells from the antennary ganglion of a 

 honey-bee just emerged in the perfect form ; 4, of 

 an old honey-bee. The nucleus is black in the 

 figure. In 3 it is very large, in 4 it is shrunken 

 and the cell-substance contains vacuoles (Hodge). 



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 grow- 

 ing out of them. The cross-section of the axis-cylinder is, and 

 remains, almost exactly equal to the area of the medullary sheath 

 (Donaldson). Even after puberty is reached the anatomical 

 organization of the nervous system may still continue to advance, 

 although at an ever-slackening rate, and the finishing touches 

 may only be given to its architecture in adult life. In old age 

 the nervous elements decay as the body does. The cell-body 

 diminishes in size ; the stainable material lessens in amount ; 

 vacuoles form in the protoplasm and pigment accumulates ; the 



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