THE TISSUES 85 



no heat production, and no phenomena of fatigue can be 

 demonstrated. 



4. The function of the cell is to preside over the nutrition 

 of the neuron. If any part of the neuron is cut off from its 

 connection with the cell, it dies and degenerates (see p. 173). 



In the cat, excitability disappears after three days, and the 

 white sheath shows degeneration changes in eight days. The 

 fatty matter runs into globules and stains black with osmic 

 acid even after treatment with chrome salts (Marchi's method). 

 This seems to be due to the fact that osmic acid acts upon 

 the unsaturated oleic acid, and that in the normal nerve this is 

 oxidised by the chrome salt, whereas in the degenerated nerve 

 so much is set free that it cannot all be oxidised, and therefore 

 stains with osmic acid. The white substances gradually 

 disappear. At the end of a month the phosphorus has all gone, 

 and by the end of about forty-four days the fat can no longer be 

 detected. At this stage Marchi's method is useless, and the 

 degenerated fibres may be demonstrated by the fact that they 

 do not stain with osmic acid or with Weigert's hsematoxylin 

 method, which stains the white sheaths of normal fibres. As 

 the degeneration advances, the axis cylinder breaks down and 

 the nerve corpuscles proliferate and absorb the remains of the 

 white sheath, so that nothing is left but the primitive sheath 

 filled by nucleated protoplasm. Into this, axons may grow 

 downwards from the central end of the nerve, and regeneration 

 may occur. This generally begins after forty-four days and is 

 well marked after about one hundred days. 



Some investigators have maintained that regeneration occurs 

 by the development of new fibrils in the degenerated nerve 

 itself, but the mass of evidence indicates that when an apparent 

 peripheral regeneration has occurred it has been due to the 

 ingrowth of axons from adjacent cut nerves. 



The cell of the neuron appears to have the power of ac- 

 cumulating a reserve of material as Nissl's granules, for it 

 has been found that after continued action these granules 

 diminish in amount. The nucleus, too, would seem to have 

 the power of giving off material for the nourishment of the 

 neuron, since in conditions of excessive activity it has been 

 found shrunken and distorted. 



