GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 241 



of others, but only come in close contact with them. They are 

 contiguous but not continuous, and the nerve impulses pass from 

 the one to the other by contact rather than by transmission 

 through continuous tissue. 



Every nerve cell gives off at least one process called the axon, 

 and it is this which forms the axis cylinder of the nerve fiber. 

 There are usually other processes, but they differ from the axon 

 in that they branch freely and do not run for any distance from 

 the cell. They are called dendrites. The axon may also occasion- 

 ally give off a branch, often called a collateral, but it is not until 

 it has reached the effector organ or some other nerve cell that 

 the branching is pronounced. It now breaks up into a mass of 

 fine branches. When these occur at a second nerve cell, they 

 closely encircle the cell, forming a basket-like structure around 

 it. This is called a synopsis. The nerve impulse can travel from 

 the fiber through its synapsis on to the nerve cell, which this sur- 

 rounds, but it cannot travel in the opposite direction. This valve- 

 like action at the synapsis explains why a nerve impulse travels 

 along a reflex arc in one direction only. Each nerve cell with 

 its axon and dendrites is called a neurone. Reflex arcs are there- 

 fore composed of two or more neurones, and the nervous system 

 is built up of great numbers of reflex arcs. 



The nerve cells which constitute the centers are usually col- 

 lected in groups called ganglia. In the segmented invertebrates, 

 such as the worms and crustaceans, there is one such ganglion 

 for each segment, each ganglion being connected with its neigh- 

 bors by nerve fibers, thus forming a chain along the ventral 

 aspect of the animal, and also having numerous nerve fibers con- 

 necting it with the various receptors and effectors of the segment 

 (Fig. 40). At the head end of the animal several of these gang- 

 lia become fused together to form a larger ganglion, which lies 

 just behind the gullet and from which two fibers pass around the 

 gullet to unite in front of it in a large ganglion, which usually 

 shows three lobes. These larger head ganglia receive the affer- 

 ent nerve fibres from the adjacent projicient sense organs, 

 namely, the eyes, the ears, the organ of smell, and the antennas 

 or feelers; these being really receptors which have become 



