THE CHIEF CENTER OF THE NERVOUS SYSTEM. 593 



mechanical stimulation of this organ." To ascertain the iden- 

 tity of the agent which gave rise to these results de Cyon had 

 the extract analyzed by as competent a chemist, M. Rossbach, 

 as he is himself a physiologist, and ascertained that the active 

 principle was phosphorus, evidently that of the only organ of 

 the two which Howell found to be active: i.e., the posterior 

 pituitary body. It seems clear, therefore, that the posterior 

 pituitary body and the anterior pituitary ~body are loth centers for 

 the conversion of chemical energy into nervous energy, and that 

 the posterior pituitary, being mainly composed of neurons and 

 their protoplasmic extensions, is the seat of reactions similar to 

 those that prevail in other neurons. 



The intrinsic processes upon which the physiological 

 functions of neurons and nerves depend seem to us to be rep- 

 resented in the foregoing pages, but we have still to account 

 for the "stormy processes in the nerve-fiber" to which Barker 

 refers: i.e., the exacerbations through which passive functions 

 become active. Can we attribute these to the cells in the sev- 

 eral centers? "Notwithstanding almost infinite minor varia- 

 tions in form," says Professor Barker, "the neurons in the most 

 different parts of the nervous system present surprisingly 

 similar general external morphological characteristics." We 

 have seen, by the details furnished by the histological studies 

 of Berkley, that such is not the case with the neurons in the 

 posterior pituitary. Indeed, there are in this organ ten cells, 

 exclusive of four of the neuroglia type that differ in morpho- 

 logical characteristics, each of which receives from Berkley a 

 separate description. We have seen with what significant reg- 

 ularity all the axons of the cells in this lobe point upward: 

 i.e., toward the infundibulum. That a similar diversity of cel- 

 lular shapes occurs in the latter is beautifully illustrated in the 

 plate of Berkley's shown opposite page 594, and representing a 

 diagrammatic drawing of a transverse vertical section of the in- 

 fundibular region. 



We have seen that above the infundibulum i.e., in the 

 structures of the third ventricle he found "varieties of epen- 

 dymal neuroglia-cells, previously supposed to have entirely 

 disappeared from the central nervous system," etc., and which 

 were thought to be confined to "reptiles, amphibia, and fishes." 



