THE NERVE CELL. 

 By E. A. Schafer. 



Contents. — Classification of Nerve Cells, p. 592— Theory of Isolated Units (Neu- 

 rone Theory), p. 593 — Structure of Nerve Cells, p. 594 — Changes in Nerve Cells 

 as the result of Excitation, p. 599 — Changes resulting from Section of the 

 Nerve Fibre Process, p. 600 — Conduction in Nerve Cells, p. 605 — Time lost in 

 Transmission from Cell to Cell, p. 608 — Refractory Period, p. 614. 



Nerve cells may be broadly classified into — (1) afferent cells, which 

 receive impressions at the periphery, and convert such impressions into 

 impulses which pass towards the central nervous system ; (2) efferent 

 cells, which send out nervous impressions towards the periphery ; 

 (3) intermediary cells, which receive impressions from afferent cells and 

 transmit them directly or indirectly to efferent cells ; (4) distributing 

 cells, which occur near the periphery, and, receiving impulses from 

 efferent cells, distribute them to involuntary muscles and secreting 

 organs. The cells classed under (1) and (2) are also known as root cells. 

 The greater number of the nerve cells of the brain and cord belong to 

 the intermediate class ; they serve the purposes of association and 

 co-ordination, and afford a physical basis for psychical phenomena; 

 the cells classed under (4) belong to the so-called sympathetic 

 system. 



Originally the afferent or sensory root-cell is situated at the peri- 

 phery, in some part of the integument, and is connected with the central 

 nervous system by a process (nerve-root fibre) which ramifies amongst 

 the nerve-cells of the central organ. This primitive condition is found 

 in the earthworm, and it is also persistent in the olfactory cells of all 

 vertebrates. In the process of evolution the peripheral cell sinks below 

 the integument, leaving a process at the surface. Ultimately the body 

 of the cell is found close to the central nervous system (in the spinal 

 ganglion), and the peripheral process is found to have developed into the 

 peripheral sensory nerve (Fig. 300). The typical efferent root cell is 

 the cell of the anterior horn of the spinal cord, which sends out a 

 nerve fibre directly to one or more muscle fibres or gland cells. 



Every nerve fibre is a process of a nerve cell. As was first made 

 clear by the researches of Deiters, 1 most nerve cells have, besides the 

 nerve fibre or axis cylinder process (neuron, 2 axon), an uncertain number 

 of other processes (dendrons), 3 which differ from the nerve fibre process 



1 "Untersuch. u. Gehirn. u. Riickenmark," Braunschweig, 1865. 



2 From vevpov, a nerve. This term was suggested by me (Brain, London, 1893, vol. xvi. 

 p. 134) to denote that the process in question is a nerve fibre, and I regret that only one or 

 two prominent neurologists have employed it in this sense. The terms neuraxon, neurite, 

 are also used by authors to express the same idea. 



3 Dendrites of some authors. 



