^12 THE CELL AND MAMMALIAN HISTOLOGY 



into close contact with dendrons or the cell body of other neurons. 

 The central nervous system consists mainly of masses of such 

 cells making a solid network (Fig. 415). There are also non- 

 nervous packing cells called collectively neurogUa ; they are 

 derived from the same embryonic epithelium as the nerve cells. 

 They occupy all the space between the nerve cells, so that they 

 make an internal connective tissue. There is much evidence, 

 both anatomical and functional, that there is no protoplasmic 

 continuity at the point, called a synapse, where one neuron 

 makes contact with another. 



NERVE FIBRES 



Many axons leave the central nervous system and each in 

 doing so becomes the centre or axis cylinder of a nerve fibre. 

 The other parts of this are shown in Fig. 399. The endoneurium 

 is collagenous and so presumably formed by connective tissue 

 cells (see p. 514). The Schwann cells, like the neuroglia, are derived 

 from the same embryonic epithelium as the nerve cells. It is 

 possible, and theoretically probable, that the Schwann cytoplasm 

 extends over the whole length of the fibre, but if so it is very thin. 

 The myelin or medullary sheath is a layer of fatty material which 

 is not present in young or small fibres, and it is the blackening 

 of the fat which gives the characteristic appearance to osmium 

 tetroxide preparations of nerves. At intervals the sheath is broken 

 and the axon is visible ; these breaks are called nodes of Ranvier. 

 The fact that there is one Schwann nucleus in each internode, 

 and other evidence, suggests that the Schwann cells control 

 the deposition of the myelin. Careful staining shows that the 

 axon has many longitudinally running neurofibrils, which are 

 also visible in the bodies of nerve cells. From the Protozoa 

 upwards, cytoplasm which is concerned in conduction tends to 

 have a fibrillar structure. Nervous impulses may be carried 

 in any direction in the cells, but the synapses have one way 

 conduction only, so that in life there is directed flow through the 

 nervous system. In some sense organs, such as the eye and nose 

 (p. 534), nerve cells are arranged in plates of tissue which simulates 

 an epithelium, and is often referred to by that name. 



