200 NERVE-CENTKES. [CH. xvn. 



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 methylenc 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 JTissl's granules, are also stained blue. These 

 extend some distance into the deudrous, but riot into the axou. 

 The substance of which they are composed is termed chromato- 

 plasm, or chromophilic material. The existence of granules in 

 cells which have an affinity for basic dyes like 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 nucleo-proteid. 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 eell. 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, and during the act of secretion they are 

 discharged and converted into constituents of the secretion." It 

 is stated by some observers that the Nissl granules are used up 

 during the discharge of energy from nerve-cells, and it certainly 

 is the case that if the cells are examined after an epileptic fit, in 

 which there has been a very massive discharge of impulses, the 

 Nissl granules have disappeared, or at least broken up into fine 

 dust-like particles, so that the cell presents a more uniform blue- 

 staining. It is, however, doubtful whether this is due to a trans- 

 formation associated with intense activity, or whether it may not 

 be caused by venosity of the blood. The cells are very sensitive 

 to altered vascular conditions ; amernia, for instance, produces a 

 similar change accompanied with swelling of the cell, and swelling 



