144 



HISTOLOGY 



Dendrites. 



Cell body. 



Collaterals. 



Medullary sheath. 



Neurolemma. 



have not developed. The dendrites have the granular structure of the 

 protoplasm from which they grow out, and were therefore originally 



named "protoplasmic processes." The neur- 

 axon, although receiving delicate fibrils from the 

 protoplasm, as shown by special methods, seems 

 quite distinct from the cell body. At its origin 

 it often appears as a clear slender cone, free from 

 granules, implanted directly upon the cell body, 

 or upon the root of one of the larger dendrites. 

 It tapers as it passes outward, and its fibrils 

 come close together so that they appear to unite. 

 Beyond the apex of the cone, which is a place 

 where the neuraxon is easily broken, the fiber 

 enlarges and its constituent neurofibrils spread 

 apart so that they are more readily distinguish- 

 able. They are imbedded in a fluid interfibrillar 

 substance. The neuraxon may send out col- 

 lateral branches, which are usually at right angles 

 with the main fiber. 



As the neuraxon passes out from a motor cell 

 it is at first free from any surrounding sheath 

 (Fig. 131, a). In the outer layer of the spinal 

 cord it becomes coated with a layer of the re- 

 fractive fatty substance known as myelin. This 

 is formed in the cord or medulla spinalis, and 

 fibers which have this sheath are said to be 

 medullated fibers (Fig. 131, b). The cells of the 

 neuroglia network, through which the nerve 

 passes while within the cord, may take part in 

 forming the myelin, but they do not produce a 

 membrane around each nerve, and they are not 

 shown in the diagram. On leaving the cord, the 

 neuraxon is still surrounded by the myelin sheath, 

 but the latter is invested by a membrane called 

 the neurolemma or sheath of Schwann (Fig. 131, 

 c). At quite regular intervals along the course 

 of the fiber, the myelin sheath is constricted or 

 interrupted, forming the nodes of Ranmer. 

 These are 0.08-1.00 mm. apart, being closer 

 together in growing fibers, and in the distal part 

 of adult fibers Midway between two nodes 



there is a nucleus, which may be found at any point in the circumfer- 

 ence of the fiber, just within the neurolemma; it occupies a depres- 



Node of Ranvier. 



FIG. 131. DIAGRAM OF A NERVE 

 CELL. 



