1. The cells of the adrenal cortex of the cavy and mouse are sometimes innervated directly by nerve-fibres passing over and around the cells, but such nerve-fibres are very scarce in the cortex, the majority of whose cells are probably not under nervous influence.
2. There is a complex plexus of nerve-fibres among the cells of the adrenal medulla, the fibres of the plexus coming into close contact with the surface of the chromophil cells, but never penetrating inside them.
3. Formations comparable with the boutons de passage and the boutons terminaux of the central nervous system occur in this plexus.
4. However, the average number of boutons of both sorts per fifty medullary cells was found to be only 1.8 in young guinea-pigs, and 1.4 in adults. It is, therefore, concluded that they do not represent the main agent by which the secretory cells are activated, but that this function is performed by the nervous impulse passing in the network of fibres around the cell.