858 



THE CENTRAL NERVOUS SYSTEM 



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 nucleus 

 shrinks; the nucleolus is obscured or may disappear altogether. At 

 the same time the processes of the cell, and especially the dendrites, 

 tend to atrophy (Fig. 339). 



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 

 possesses one, undergo a rapid degeneration. It was long supposed 



Fig. 338. Section 

 through Half of Neural 

 Tube (Barker, after 

 His). The pear- 

 shaped neuroblasts are 

 seen migrating out- 

 wards. The axons of 

 some of them are seen 

 pushing their way out 

 through the marginal 

 veil as the anterior 

 root of a spinal nerve. 



Fig. 339. 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 per- 

 fect 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). 



that no change took place in the nerve-cell. The researches of recent 

 years have shown that not only does loss of the specific function and 

 trophic influence of the cell-body affect the nutrition of the axon, but 

 loss of function of the axon reacts on the cell-body. In many cases 

 at least, when a nerve-fibre is divided from its cell, characteristic 

 changes are produced in the latter and in its dendritic processes, and 

 they are scarcely less rapid, although usually less profound, and far 

 more transient than the degeneration in the peripheral portion of the 

 nerve-fibre. The cell-body and the nucleus swell. Many of the Nissl 

 bodies (Fig. 340) disintegrate, and are reduced to a finely granular 

 condition. After a time much of the disintegrated chromatic sub- 

 stance disappears altogether. The nucleus may be displaced to one 



