THE CENTRAL NERVOUS SYSTEM. 41 



of the dendrons, but not in the axon hillock, the part of the cell from 

 which the axon takes origin. The fibrils run through the cell from 

 dendrons to axon, and, with some interfibrillar material, form the 

 substance of the latter structure. Many nerve cells have a well-marked 

 lymph capsule surrounding them. 



The axon is, as has been said, a fibrillar structure, and usually 

 becomes the axis cylinder of a nerve fibre, acquiring a myelin sheath 

 soon after leaving the cyton. Axons are of variable length, the longest 

 being those which extend from the lumbar region of the spinal cord to 

 the foot. They do not branch as the dendrons do,, but those which 

 run a course in the central nervous system give off delicate twigs 

 at right angles to their course, known as collaterals. Each axon ends 

 by a terminal arborisation, either in relation with the cyton or dendrons 

 of another neuron or in relation with muscle fibres or gland cells. The 

 termination in connection with striped muscle frequently enters into 

 the formation of a special form of nerve-ending known as an end-plate. 

 The axons of certain cells in the central nervous system, belonging to 

 what is known as Golgi's second type, are very short, do not acquire 

 a myelin sheath, and form their terminal arborisation in relation with 

 the cell body of a neighbouring neuron. 



Dendrons are found in their most typical form in the neurons of the 

 brain and spinal cord. They branch in a tree-like manner, and the 

 branches frequently exhibit minute enlargfijuents or projections. The 

 dendrons are short as compared with the axon, and never extend any 

 distance from the cyton. 



The processes of nerve cells do not anastomose, but come into 

 relationship by the more or less intimate mingling of the terminal 

 arborisation of the axon of one neuron with the dendrons or cyton 

 of another neuron. Such a communication, in which there is contact 

 without continuity, is called a synapse. The contact is possibly not 

 direct, the transmission of impulses from one neuron to the other being 

 effected through an intermediate layer of some substance which does 

 not form part of either neuron. 



In addition to the fibrils which have been described as occurring in 

 the substance of each nerve cell, an extracellular network of fibrils has 

 been described, but it has not been established that there is continuity 

 between the two networks. A third fibrillar network has been 

 demonstrated, chiefly in invertebrate animals, which is said to be 

 continuous from neuron to neuron, and, on the basis of this description, 

 a theory has been put forward that nerve impulses are transmitted 

 through the central nervous system by means of a continuous network 

 of fibrils and not by a series of synapses. The evidence upon which 



