THE PROPERTIES OP EACH PART OF THE REFLEX ARC 801 



is composed of a viscous fluid full of extremely minute granules, each of 

 which apparently consists of a colloidal solution surrounded by a lipoid 

 envelope (Fig. 210). When the temperature is raised, the granules dis- 

 appear, and when the cells are deprived of oxygen, the cytoplasm and 

 nucleus become swollen. A similar swelling of the cell and nucleus super- 

 venes upon section of the axon; and in stained specimens the Nissl 

 granules disappear and the protoplasm stains diffusely (chromatolysis). 

 In embryonic life the processes of the nerve cells appear to be capa- 

 ble of undergoing a certain amount of ameboid movement, and fibers 

 grow out from them, indicating, therefore, that in the development 

 of the nervous system the nerve cells appear first, and the nerves sub- 

 sequently grow out from them to their proper destination. Prolifera- 

 tion of isolated tissue cells in vitro has been observed for many other 



Fig. 210. Living nerve cells (from the dorsal root ganglia of a dog three days old) examined 

 by the ultramicroscope. There are no Nissl bodies or neurofibrils, only fine particles, present in 

 the protoplasm. (From Marinesco.) 



tissues, such as cardiac muscle, renal epithelium and connective tis- 

 sue. Its occurrence indicates that the therapeutic principle that the 

 aim of treatment should be to give the diseased organ a rest so that by 

 cell regeneration it may recover its lost function, is one which may ap- 

 ply to the nerve tissues of young animals. Whether adult nerve cells 

 may regenerate is as yet not certain. 



This growing out of nerve fibers from their cells is the essential na- 

 ture of the development of the nervous system in the developing animal. 

 At birth, unlike the cells of other tissues, those of the central nervous 

 system are already provided. No new ones are added during postnatal 

 life. The axons gradually develop from this inherited stock of nerve 

 cells, and by connecting with other neurons serve to bring about the 

 integration which is the important characteristic of the adult nervous 



