24 



INTRODUCTION 



A nerve cell (fig. 15) is uni-, bi- or multipolar accordingly as it 

 has one, two or more processes. In unipolar cells (C) the single 

 process sooner or later divides, so that the cell in reality is at least 

 bipolar. At either end the processes may break up into minute 

 twigs, or at one end may terminate, as in muscles and sensory 

 structures, in special end organs. Apparently the impulse enters 

 the cell at one side by one or several processes, and is carried out by a 

 single process. Thus there are to be recognized afferent and efferent 

 processes. The efferent process is called the axon (axis cylinder), 

 When the axon breaks up into twigs, these are called telodendra. 

 similar twigs on the afferent sides being dendrites (fig. 15, A, t and d). 



All parts of the neuron proper — cell bodies and processes — are 

 gray in color, but usually the processes are surrounded by a medullary 

 sheath (w) of a peculiar white substance (myelin) rich in fat, which 



Fig. 15. — Various kinds of nerve cells. A, multipolar cells; B, portion of nerve 

 fibre with sheaths; C, unipolar cell; D, pyramidal cell; a, axon; c, collateral; d, dendrites; 

 cb, cell body; m, medullary sheath; «, nucleus of cell of Schwann's sheath; r, node of 

 Ranvier; s, sheath of Schwann; t, telodendron. 



apparently acts in part as an insulator, preventing nervous impulses 

 from passing from one axon to another. It is also nutritive in char- 

 acter. This sheath does not continue over the dendrites. Fre- 

 quently the dendrites of two neurons interlace for the transference of 

 stimuli from one to the other, but the present opinion is that, at least 

 in vertebrates, there is no actual continuity of substance between 

 neurons, only an interlacing of terminal twigs. The medullary sheath 

 is not cellular, but frequently fibres may be surrounded by a sheath 

 of Schwann {s), with scattered nuclei. This has been regarded as 

 mesenchymatous, but recent researches tend to show that it is 

 ectodermal, its cells coming from the early nervous system. 



Nervous tissue consists of these neurons plus connective tissue and 

 glia cells. A nerve, as found in dissection, consists of numbers of 



