CENTRAL NERVOUS SYSTEM. 191 



active changes occur, since the network of blood-vessels is most dense about 

 them, indicating that the metabolic processes are here most active 1 (Fig. 80). 



Chemical Changes. — For the direct microchemical determination of 

 special substances within the nerve-cells there are but few methods, though 

 some phosphorus-bearing substances (nucleins) can be demonstrated, 2 and the 

 occurrence of chemical changes due to activity and to age are very evident. 

 Macallum 3 has demonstrated the presence of iron in the stainable substance 

 of Nissl. There is general consensus that the alkalinity of the nerve-tissues 

 is decreased during activity, and this decrease in alkalinity may amount at 

 times to a positively acid reaction. This change, too, is better supported by 

 the observations made where the cell-bodies are numerous than by those 

 made where the fibres are alone present. 



Fatigue. — Not only is the food-supply to the nerve-cells, as represented 

 by the quality and quantity of the plasma, variable, but the cells themselves 







Fig. 80.— Frontal section through the human mid-brain at the level of the anterior quadrigeminum 



(Shimamura). On the left side the blood-vessels have been injected; on the right the gray matter is 

 indicated by the heavy lines. It appears by this that the blood-vessels are most abundant in the gray 

 matter. 



are subject to wide variations in their power to utilize these food materials, 

 and deviations from the normal in either of these respects means a diminu- 

 tion in the physiological powers of the cell, which we may call fatigue. In 

 the nervous system the signs of fatigue are both physiological and histological, 

 but it is to the latter changes only that attention will be here directed. 



If a faradic current is applied intermittently to the mixed nerve-trunk 

 going to a limb, changes are to be observed in the cell-bodies belonging to 

 the spinal ganglia of the several roots forming the nerve (Hodge). 



When this experiment is made on a cat, and, alter death, the sections 

 from the stimulated are compared with those of the corresponding, but mi- 

 stimulated, spinal ganglion, a picture like that represented by Fig. 81, is 

 obtained. 4 



1 Shimamura: Neurologisches CerUralblalt, 1894, Bd. xiii. 



! Lilienfeld and Monti : Zeitschrift fur physiologische Chemie, L892, Bd. xvii. 



3 Macallum : British Medical Journal, London, 1898, vol. ii. p. 778. 



4 Hodge : Journal of Morphology, 1S92. 



