22 ATLAS OF NERVE CELLS 



together, and it is supposed that an activity in one may excite activity in others, through the 

 medium of neuraxons and dendrites either by accidental contact, or, more probably, by the 

 induction of impulses in the one from the other; just as an electric current in one wire will 

 set up a current in independent wires about it. 



Plate IV. shows a large motor cell of the anterior horn of the spinal cord of a foetal 

 pig, with numerous dendrites. The interlacing of the dendrites is particularly evident in the 

 plate, where many dendrites belonging to cells not shown in the plate intertwine with those 

 whose origin is shown. Gerlach believed that the sensory nerve fibres after entering the cord 

 became continuous with the ends of the dendrites, and that thus sensory impulses are sent 

 into nerve cells. But this theory has been abandoned, and the free termination of all den- 

 drites must be accepted. There is no continuity between nerve fibres and dendrites. There 

 is no real network of nerve fibres in the nervous system. The sensory impulses come in, as 

 we shall soon see, by nerve fibres which terminate in free ends like brushes, and their brush- 

 like expansions may surround the nerve cells or may interlace with their dendrites, but do not 

 necessarily come into contact with them. In Plate V. just below the cell such a free ter- 

 mination is to be seen, there being little knobs on each terminal filament. 



THE NEURAXON is the second variety of branching process of the nerve cell. It was 

 formerly known as the axis cylinder projection or the functional process of the cell. The 

 neuraxon may come off directly from the body of the cell or from a small pyramidal projec- 

 tion of the body as in Plate IV., or it may arise from one of the large dendrites near the 

 cell body. The characteristics of the neuraxon which serve to distinguish it from the den- 

 drites are its uniform size from its origin onward, the possession of a myelin sheath giving 

 it a smooth surface, and its lack of many branches. It was formerly supposed that the axis 

 cylinder process never branched. This, however, is not so. The neuraxon gives off branches 

 called collaterals, but these differ greatly from the branches of the dendrites. In the first 

 place, the branches of the neuraxon are usually given off at right angles to its direction; in 

 the second place, they are always very small and fine as compared with the neuraxon. There 

 is often a slight swelling of the neuraxon at the point of origin of a collateral, as may be 

 seen in Plates IV. and VI. In Plates III. to VIII. numerous neuraxons can be seen with fine 

 collaterals coming off from them. They are seen to interlace with the dendrites, but their 

 smaller size, regular smooth contour, and method of giving off branches, nearly at right angles, 

 serve to distinguish them from dendrites. Golgi considers that it is only through the neu- 

 raxon that the cell performs its function, but this, as already stated, is not accepted. Van 

 Gehuchten and others have agreed that all neuraxons are cellulifugal in function, conveying 

 impulses outward from the cell, but this too is not fully established. 



