Page:The Harveian Oration,1902.djvu/31

 tion, which is kept up mainly by the nervous system. The centre which governs the calibre of the blood-vessels is situated in the pons-medulla, probably in the group of cells known as the antero-lateral nucleus—the homologue of the cells of the intermedio-lateral tract of the spinal cord (Gaskell). Hence on section of the cord below this point there is extensive vaso-dilatation, with an enormous fall in the blood pressure. But though this is the chief centre of vaso-motor regulation, there is evidence that there are also subsidiary centres in the cord itself For, after section below the medulla oblongata, the vessels after a time regain a considerable amount of tone, and reflex alterations in their calibre can be elicited by appropriate stimuli. The blood-vessels also possess an intrinsic activity similar to that of the heart itself, and exhibit slow rhythmical variations in calibre; and the recent experiments of Bayliss prove that they respond to mechanical variations in internal pressure — constricting or relaxing, with a rise or fall respectively. The antagonistic nerves, the vaso-dilators, the homologues of the cardiac