An experimental study of the fornix in the rabbit.

The extrinsic connexions of the hippocampus have been studied in a number of vertebrate types, and the complexity of the system has been described by many workers (cf. von Kolliker, 1894, 1896; Elliot Smith, 1896; Cajal, 1911; Johnston, 1913; Kappers, Huber & Crosby, 1936; Young, 1936). The voluminous and controversial literature based on normal material has been reviewed by these authors and will not be repeated here. The experimental papers are few (cf. Edinger & Wallenberg, 1902; Gudden, 1881; Probst, 1901; Allen, 1944; Fox, 1943; Gerebtzoff, 1941-2), and these are based chiefly upon the Marchi method and, to a lesser extent, upon studies of retrograde cell degeneration. The information thus gained, however, is incomplete and contradictory, and there is thus need to undertake a re-examina-tion of this system with modern methods. The present study reports the results of experiments in rabbits in which small lesions have been placed in various parts of the fornix system (Text-fig. the brains being subsequently prepared by a modification of the silver method of Glees This technique demonstrates in very satisfactory fashion the degenerative frag-mentation of nerve fibres and their terminals, whether the latter consist of boutons terminaux or a pericellular plexus (free endings). The terminals may show marked degenerative changes as early as the third day following section of the axons, though in the case of the larger fibres these may not be apparent until the fifth day. During this period the degeneration is wholly distal to the site of the lesion with the exception only of retraction bulbs which are developed at the cut ends of the fibres. The method forms, without question, the most reliable, sensitive and precise technique for the study of fibre degeneration following axonal interruption within the central nervous system which is available at present, although isolated fibre connexions of minor significance are difficult to follow and