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A MANUAL OF PHYSIOLOGY 



the central canal of the cord and the ventricles of the brain has also 

 been considered by some as of nervous nature ; but in spite of the 

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

 which pierce far into the grey substance, which has been supposed 

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



Growth of Neurons. The growth of a neuron is a comparatively 

 slow process. Early in foetal life (about the third or fourth week in 

 man) certain round germinal cells make their appearance amid the 

 columnar epiblastic 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 axon or axons, and then the dendrites 

 (Fig. 229). 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 ner- 

 vous elements of the brain and cord, 

 ultimately acquire a medullary sheath, 

 although, as we have said, the time at 

 which medullation 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 inti- 

 mate than they will afterwards become. 

 For many years the processes, and par- 

 ticularly the axons, continue not only 

 to grow longer, but also to grow thicker. 

 FIG. 232,-PERicELLULAR DAS- The cell-body also enlarges, and the 

 KETS (SCHAFER, AFTER CAjAL). quantity of material in it that stains 

 Two cells of Purkinje from the with basic . d y es increases. Even after 

 cerebellum are seen surrounded by puberty is reached the anatomical 

 end ramifications forming a basket- organization of the nervous system may 

 work> b: *' a> 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 stain- 

 able material lessens in amount ; vacuoles form in the protoplasm 

 and pigment accumulates ; the nucleus shrinks ; the nucleolus is 

 obscured or may disappear altogether. At the same time the pro- 

 cesses of the cell, and especially the dendrites, tend td atrophy 

 (Fig. 234). 



Nutrition of the Neuron. We have already seen that when an axon 

 is cut off from its cell-body, it and its medullary sheath, when it pos- 

 sesses one, undergo a rapid degeneration. It was stated by Waller, and 

 was long supposed to be true, that no change took place in the nerve- 

 cell. The researches of recent years have shown that not only does 

 loss of function of the cell-body affect the nutrition of the axon, but 



