The retrograde transport of horseradish peroxidase was used to study the topographic and laminar origins of the cortical projections to the parvocellular and the magnocellular divisions of the red nucleus in Macaca mulatta and Macaca fascicularis. Approximately 90% of the corticorubral projection is directed to the parvocellular division of the nucleus. Corticoparvocellular (CRp) neurons are pyramidally shaped, are smaller in size than corticospinal neurons, and are more numerous. They are found principally in sublamina Va of cytoarchitectonic areas 4 and 6, and in moderate quantities in sublamina Vb of posterior area 8 and area 5. In areas 4 and 6, the cells are grouped in clusters of three to 15 neurons each and are arranged in cellular bands of varying rostrocaudal thickness which course mediolaterally. With respect to functionally defined zones, CRp neurons are found throughout the supplementary motor area and the precentral motor cortex. In addition, they are found in parts of areas 5, 6, and 24 that project to these cortical motor areas, and that are thought to have "premotor" or movement-programming functions. The corticomagnocellular (CRm) projection arises principally from cells in sublamina Vb of the precentral arm and leg areas (area 4), and from adjacent parts of posterior area 6, CRm cells are pyramidally shaped, and their size distribution is bimodal, with peaks that correspond, respectively, to the modal diameters of CRp and of corticospinal neurons. These results and those of previous studies suggest that CRm neurons are involved principally in the control of hand and foot movements, with little effect on more proximal musculature. The massive CRp projection, however, is clearly part of a large cerebrocerebellar communication system, with motor and/or movement programming functions that have yet to be clearly defined.

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1. The regulation of cerebrospinal fluid (c.s.f.) bicarbonate concentration was studied using the cat choroid plexus isolated in a chamber in situ. 2. Decreases in plasma bicarbonate concentration caused relatively small changes in the c.s.f. bicarbonate concentration. 3. Alterations in c.s.f. bicarbonate concentration (c.s.f. HCO3‐=9 or 28 m‐equiv/l.) were countered by changes in the bicarbonate concentration of the fluid produced by the plexus or in the rate of bicarbonate transport which returned c.s.f. bicarbonate towards normal. 4. There was significant regulation of pH in the choroid plexus fluid during hypocapnia and hypercapnia. 5. Alterations of plasma acid‐base status did not significantly alter the potential difference across the choroid plexus. However, the potential difference increased when c.s.f. bicarbonate was increased and decreased when c.s.f. bicarbonate was decreased. 6. The data indicate that the bicarbonate concentration in the c.s.f. is actively regulated by the choroid plexus during acid‐base disturbances occurring either systemically or in the c.s.f.

SUMMARY1. A method was developed for isolating a segment of the choroid plexus of the lateral ventricle of the cat brain in a chamber in situ.2. A comparison of electrolyte and protein concentrations in serum, ultrafiltrate of serum, cisterna magna fluid, fluid accumulated in the chamber and fluid collected from the choroid plexus by another technique, demonstrates that the chamber fluid is a secretary product of the choroid plexus.3. The rate of fluid formation in the chamber was 0 4 10. min-mg-' of tissue, a value in good agreement with reports in the literature.4. The observation that the concentration of K+ in choroid plexus fluid was lower than that in the serum ultrafiltrate suggests that K+ is regulated by an active transport process at the choroid plexus.5. Significant correlation was found between electrolyte values and the protein content of the chamber fluid. This suggests that plasma is the probable source of the protein in the chamber fluid and that K+ and probably Ca2+ and Mg2+ in c.s.f. are subject to active homoeostatic regulation by the choroid plexus. 6. The technique described provides a new opportunity to study in detail the functional characteristics of the choroid plexus.

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