THE NERVOUS SYSTEM 



437 



to the end of the body. The majority of the neurones originating 

 in the brain are no longer than the nearest cell station, such as 

 the basal ganglia and medulla ; as a rule, either in one or the 

 other, or in both, of these places the neurone ends by arborising 

 around a cell in a ganglion, and a fresh neurone is formed. Even 

 when it gets into the cord it has not necessarily a long run ; it 

 may run the length, but the majority of the cord neurones do not ; 

 they dip into the grey matter at intervals, and arborise around 

 cells from which fresh fibres arise. This may be the last cell 

 station or not ; if it is the last, the axon passes out into a peripheral 

 nerve, but even when outside the spinal cord a fresh break may 

 occur — as we shall see in dealing with the sympathetic system — 

 more arborising around cells in the sympathetic ganglia, and 

 finally new fibres. This is what is meant in speaking of the 

 neurones being arranged end to end like the links of a chain. In 

 the simplest conceivable form of nervous mechanism the smallest 

 number of neurones is two ; it is seldom that any such simple 

 number exists in the body mechanisms ; as a rule, there are several 

 breaks between the periphery and the brain, or the brain and the 

 periphery. A single motor cell in the cerebrum, with its axon, is 

 not connected with a single spinal motor neurone ; the latter may 

 be connected with many such cerebral neurones. The sum of all 

 the fibres in the white substance of the brain is, therefore, 

 obviously larger than the sum of those in the cord. 



The conducting path is always from dendrites to cell, and 

 from cell to axon. In a neurone a stimulus can only be trans- 

 mitted in one direction — viz., from dendrite to axon, so that the 

 dendrite is the receiving portion of the cell, the axon the issuing 

 or distributing portion. No matter how many neurones the 

 impulse has to pass through, nothing is lost excepting time. It 

 is delayed, but it is carried from the termination of the axon 

 across unoccupied fields to the nearest dendrites, and so trans- 

 mitted to the next cell. Xhere is no contact of the cellular 

 elements of one axon with those of another. This is an essential 

 part of the neurone doctrine. There is contiguity, but no structural 

 continuity. 



Such is the principle existing everywhere in the nervous 

 system, brain or spinal cord, afferent or efferent nerves, cerebro- 

 spinal or sympathetic. There may be variations, and here and 

 there the general rules of construction may be slightly departed 

 from, but the principle is maintained — a principle which explains 

 not only the anatomical but the physiological side of the system, 

 and enables the mind to grasp a scheme of unity of construction 

 throughout the entire nervous system. 



The cell is the life of the fibre. If the fibre be cut off from the 

 cell, it degenerates. Nor is the nutritional change confined ta 



