On the occurrence of a mesocælic recess in the human brain, and its relation to the sub-commissural organ of lower vertebrates; with special reference to the distribution of Reissner’s fibre in the vertebrate series and its possible function

Abstract

Eight years ago one of us (Dendy, 1902) published in the ‘Proceedings’ of the Royal Society a description of a peculiar structure lying beneath the posterior commissure in the brain of the Ammocœte, in the shape of a pair of longitudinal grooves lined by greatly elongated and apparently ciliated columnar cells, and evidently formed by specialisation of the ependymal epithelium which lines the cavity of the brain. Previous to this time there appear to have been, at the most, only a few scattered references to the occurrence of any such structure in the vertebrate brain. We now know, however, that it occurs throughout the whole vertebrate series, from the lampreys to the primates it having recently been figured in the case of Macacus , by Sir Victor Horsley (1908). Sargent (1908) has given to this structure the name “Ependymal Groove,” and has described it in a number of Ichthyopsidan types. He says, however (1903, 1904), that it is inconspicuous in mammals. We ourselves have recently observed it in a number of different forms. It is very well developed, for example, in Sphenodon, and we find it also strongly developed in the mouse and the cat. Sargent showed, further, that the “ependymal groove” is intimately connected with the anterior end of Reissner’s fibre, and in fact regarded it merely as a kind of attachment plate for the latter. We also have been able to demonstrate quite clearly its connection with Reissner’s fibre in a large number of types, e. g. Geotria (Dendy, 1907) and Rana (Nicholls, 1908). We shall discuss its possible function later on in the present communication; but, in the meantime, we may state that we do not consider that the term “ependymal groove” is sufficiently distinctive for so remarkable and constant a feature of the vertebrate brain, for it is, of course, not the only ependymal groove present. Inasmuch as it lies beneath the posterior commissure, we propose to speak of it in future as the “Sub-Commissural Organ.” It appears primarily to be made up of two bands of columnar epithelium, usually more or less completely united together in the form of a groove; but, whereas it remains throughout life in a well developed condition in all the lower vertebrates, in man, as we shall endeavour to show in the present communication, it becomes reduced in the adult to a mere vestige sunk in the brain tissue at the back of the posterior commissure, but unmistakably recognisable as the homologue of the sub-commissural organ of lower types.