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AN AMERICAN TEXT- BOOK OE PHYSIOLOGY. 



sheaths before the corresponding afferent (sensory) nerves are medullated 

 (except in the ease of the vestibular branch of the auditory nerve, which is 

 medullated at the same time as the motor nerves). In the central system 

 the continuations of the afferenl pathways become medullated before the 

 pyramidal tracts, while in the cerebral hemispheres medullation of the com- 

 missural and association fibres follows immediately that of the afferent tracts. 



Ambrmin and Held 1 have also shown that when the eyelids are prema- 

 turely opened in animals born blind, such as the rabbit, dog or cat, and the 

 animal is then exposed to the light, the medullary sheaths are more rapidly 

 formed in the optic nerves exposed to stimulation than in those developing 

 normally. 



Changes in the Cytoplasm. — While the nerve-cell is passing from the 

 immature to the mature form, increasing in mass and in the number of its 

 branches, as well as acquiring its medullary sheath, it also undergoes various 

 chemical changes. The stainable substance in the cytoplasm becomes more 

 abundant at maturity and the pigment-granules increase in quantity. 2 



Fig. 74.— To show the changes in nerve-cells due to age : A, spinal ganglion-cells of a still-born male 

 child : B. spinal ganglion-cells of a man dying at ninety-two years ; n, nuclei. In the old man the cells 

 are not large, the cytoplasm is pigmented, the nucleus is small, and the nucleolus much shrunken or 

 absent. Both sections taken from the first cervical ganglion, X 250 diameters (Hodge). 



Old Age of the Nerve-cells. — But the nerve-cell, though possessing in 

 most cases a life-history coextensive with that of the entire body, eventually 

 exhibits retrogressive changes. These changes of old age consist, in some 

 measure, in a reversal of those processes most evident during active growth. 

 The cell-body, together with the nucleus and its subdivisions, becomes smaller, 

 the stainable substance diminishes and becomes diffused instead of appearing 

 in compact masses,' the pigment increases, the cytoplasm exhibits vacuoles, 

 the dendrites atrophy, and the axones also probably diminish in mass. In 

 some instances the entire cell is absorbed. Some of these factsare illustrated 

 by the observations of Hodge 4 on the spinal ganglion-cells of an old man of 

 ninety-two year- ;is compared with those of a new-born child (see Fig. 74). 

 Since the chemical and morphological variations which occur during the 

 entire growth-cycle are accompanied by variations in the physiological powers, 



1 hoc. cit., S. 222. * Vas: Archiv fur mikroskopische Anatomie, 1892. 



3 Marinesco : Revue neurologique, October, 1899, No. 20. 



1 Journal of Physiology, 1894, vol. xvii. 



