200 NERVE-CENTRES [CH. XVII. 



from these cells, fresh nerve-fibres pass to the arm-muscles, and 

 continue the impulse. This is shown in the accompanying diagram 

 (fig. 213). The cell of the cerebral grey matter is represented by 

 C. C., the pyramidal nerve-fibre arborises around the cell of the 

 spinal cord (S. C.) from which the motor nerve-fibre arises, and which 

 carries on the impulse. The spinal cord cells are thus surrounded 

 by arborisations (synapses) derived not only from the sensory 

 nerves (S), but by fibres from the upper part of the nervous system. 

 We now see how it is possible that reflex actions in the cord may 

 be controlled by impulses from the brain. 



The system of relays is still more complicated in the case of 

 sensory impulses, as we shall see later on ; the same is true for the 

 motor path to involuntary muscle, accessory cell-stations being situated 

 in the sympathetic ganglia. 



We may now return for a moment to the subject of degeneration. 

 If the nerve-fibre is cut off from its connection with the spinal nerve- 

 cell, the peripheral end degenerates as far as the muscle. 



Suppose, now, the pyramidal fibre were cut across, the piece still 

 attached to the brain-cell would remain in a comparatively normal 

 condition, but the peripheral end would degenerate as far as the 

 synapse round the spinal cell (S. C.), but not beyond. We can thus 

 use the degeneration method to trace out tracts of nerve-fibres in 

 the white matter of the central nervous system. The histological 

 change in the fibres is here the same as that already described in the 

 nerves, except that, as there is no primitive sheath, there can be no 

 multiplication of its nuclei ; there is instead an over-growth of neuroglia. 

 Degenerated tracts consequently stain differently from healthy white 

 matter, and can be by this means easily traced. 



Another method of research which leads to the same results as 

 the degeneration method is called the embryological method. The 

 nerve-fibres which grow from different groups of nerve-cells become 

 fully developed at different dates, and so, by examining brains and 

 cords of embryos of different ages, one is able to make out individual 

 tracts before they have blended in the general mass of white matter. 



We shall, however, return to this subject when later on we are 

 studying the physiology of the central nervous system in detail. 



The Significance of Nissl's Granules. 



If portions of the brain or spinal cord are fixed in absolute alcohol, 

 and sections obtained from the hardened pieces are stained by means 

 of methylene blue, the nerve-cells exhibit a characteristic appearance. 

 The nucleus and nucleolus take up the blue stain, and throughout 

 the cell body a number of angular-shaped masses, which are termed 

 Nissl's granules, are also stained blue. These extend some distance 



