CHAPTER IV 



NEURONS AND NEURON-CHAINS 



THE nervous system is composed of highly irritable cellular units, or neurons, 

 linked together to form conduction pathways. In the preceding chapter we 

 have seen that each neuron is the product of a single embryonic cell or neuro- 

 blast, and that, therefore, the nerve-cell with all its processes constitutes a gen- 

 etic unit. In the present chapter, as we examine the form and internal struc- 

 ture of the neurons and their relation to each other, we shall learn that they are 

 also the structural and functional units of the nervous system. 



Form. There is the widest possible variation in the shape of nerve-cells, 

 but all present some features in common. About the nucleus there is an accumu- 

 lation of cytoplasm which together with the nucleus forms what is often called 

 the cell body. A convenient term by which to designate the circumnuclear 

 cytoplasmic mass is perikaryon. From the perikaryon cytoplasmic processes 

 are given off, some of which may be of great length. The external form of 

 the neuron depends on the shape of the perikaryon and on the number, shape, 

 and ramification of these processes. Since the variety of forms is almost with- 

 out limit, we will content ourselves with studying a few typical examples. 



The pyramidal cells of the cerebral cortex are good examples (Fig. 23). The 

 perikaryon is triangular in form. One angle, that directed toward the surface 

 of the cortex, is prolonged in the form of a long thick branching process, the 

 apical dendrite. From the sides and other angles of the perikaryon arise shorter 

 branching dendrites, while from the base or from one of the basal dendrites 

 arises a long slender process, the axon. The characteristic features of the den- 

 drites are as follows: they branch repeatedly, rapidly decrease in size, and 

 terminate not far from the cell body. Their contour is irregular and they are 

 studded with short side branches, or gemmules, which give them a spiny appear- 

 ance. Each neuron usually possesses several dendrites, but in some types of 

 nerve-cells they are absent altogether. The axon, on the other hand, is char- 

 acterized by its uniform smooth contour, relatively small diameter, and in most 

 instances by its great length and relative freedom from side branches. It may 

 give off fine side branches, or collaterals, near its origin; and these arise at right 



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