The fine structure of the mossy fibre endings in the hippocampus of the rabbit.

The axons of the granule cells of the dentate fascia terminate in relation to certain of the pyramidal neurons of Ammon's Horn (Cajal, 1911). These fibres form a well defined bundle, the dentate-ammonic tract, which, as shown in Text-fig. 1, extends from the region of the dentate hilus where it first forms to the junction of regions C.A. 3 and C.A. 2 of Ammon's Horn. Lorente de No's (1934) classification of the fields of Ammon's Horn is used throughout this account. In Golgi preparations, these axons show some unusual appearances. These have been described in detail by Cajal (1911), who termed them mossy fibres, analogous to those found in the cerebellum. He observed that the fibres wind tortuously among the apical dendrites of Ammon's Horn neurons and that at intervals along their course show characteristic varicosities. These protoplasmic masses were seen to come into intimate contact with the dendrites or perikarya of the neurons and were considered by Cajal to be points of synaptic contact. Lorente de No (1934) was also of this opinion, but could not decide whether the structures formed part of the pyramidal neuron or of the axon terminating upon it. Cajal (1911) also found that these mossy endings could not be stained by reduced silver techniques. In this study the site of termination of these fibres has been investigated with the electron microscope and where possible the results have been correlated with those of light microscopy. A preliminary report of this work has been published (Hamlyn, 1961).