192 NERVE-CENTRES [CH. XVI. 



synapses through which it has to travel (see later, on the structure 

 of the visual and auditory mechanisms). 



The valved condition of nervous paths also explains another 

 difficulty. We have seen on p. 160 that under certain circumstances 

 a nervous impulse will travel in both directions along a nerve. Yet 

 when we stimulate the motor fibres in an anterior spinal root, the 

 only effect is a contraction of muscles ; there is no effect propagated 

 backwards in the spinal cord. No doubt a nervous impulse does 

 travel backwards to the anterior horn cells, but it is there extin- 

 guished, it cannot jump the synapses backwards, and there is no 

 negative variation to be detected in a galvanometer connected to the 

 pyramidal tracts in the cord. 



The law of axipetal conduction is no doubt true for the majority 

 of neurons. But there is at any rate one very striking exception, 

 namely, in the neurons of the spinal ganglia; here the impulse 

 passes to the body of the cell by one axon from the periphery, and 

 away from it to the spinal cord by the other. To say, as some do, 

 that the peripheral process is really a dendron because it conducts 

 impulses centripetally, is simply arguing in a circle. 



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, but the total 

 amount of chromatin present in the nucleus is not large, except in 

 embryonic nerve-cells ; throughout the cell body a number of angular- 

 shaped masses, which are termed Nissl's granules, are also stained 

 blue. These extend some distance into the dendrons, but not into 

 the axon. The substance of which they are composed is termed 

 chromatoplasm, or chromopJiilic material. The existence of granules 

 in cells which have an affinity for basic dyes such as methylene blue 

 is not at all common ; the granules in the majority of the white blood- 

 corpuscles, for instance, have an affinity for acid dyes. Micro- 

 chemical methods have shown that the main constituent of the Nissl 

 granules is an iron-containing nucleo-protein. The name kineto- 

 plasm has been given to it by Marinesco in order to express the idea 

 that it forms a source of energy to the cell. It can hardly be denied 

 that the substance of which the granules are composed, forming as 

 it does so large a proportion of the cell-contents, and made of a 

 material in which nuclein forms an important constituent, is intimately 

 related to the nutritional condition of the neuron. Some have even 

 compared it to the granular material which is present in secreting 

 cells; in these cells before secretion occurs, the granules accumulate, 



