CHAPTER X 

 NERVE TISSUE 



The Neurone 



In most of the cells thus far described the protoplasm has been 

 confined to the immediate vicinity of the nucleus. In the smooth 

 muscle cell was seen an extension of protoplasm to a considerable 

 distance from the nuclear region, while in the connective-tissue cells 

 of the cornea the protoplasmic extensions took the form of distinct 

 processes. Processes, often extending long distances from the cell 

 body proper, constitute one of the most striking features of nerve- 

 cell structure. Some of these processes are known as nerve fibres; 

 and nerve tissue was long described as consisting of two elements, 

 nerve cells and nerve fibres. With the establishment of the unity of 

 the nerve cell and the nerve fibre, the nerve cell with its processes 

 was recognized as the single structural unit of nerve tissue. This 

 unit of structure is known as a neurone. The neurone may thus be 

 defined as a nerve cell with all of its processes. 



In the embryo the neurone is developed from one of the ectoder- 

 mic cells which constitute the wall of the primitive neural canal. 

 This embryonic nerve cell, or neuroblast, is entirely devoid of proc- 

 esses. Soon, however, from one end of the cell a process begins to 

 grow out. This process is known as the axone (axis-cyUnder process, 

 neuraxone, neurite). Other processes appear, also as outgrowths of 

 the cell body; these are known as protoplasmic processes or dendrites. 



Each adult neurone thus consists of a cell body, and passing off 

 from this cell body two kinds of processes, the axis-cylinder process 

 and the dendritic processes (Fig. 74). 



An important exception to this rule is presented by the cells of the cerebro- 

 spinal ganglia (Fig. 300). Here the typical neurone has two processes, a periph- 

 eral process, and a central process which enters the central nervous system. 

 Both processes have a typical axis-cylinder structure. Some authorities have 

 interpreted the peripheral process as a modified dendrite. 



I. The Cell Body.- — ^Like most other cells, the nerve cell body 

 consists of a mass of protoplasm surrounding a nucleus (Fig, 75). 



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