Page:The Harveian Oration,1902.djvu/35

27 they are related to the segments of the cord, from which they respectively arise. This at least is true of the nervi erigentes, for when the cord is divided above their origin their function can be excited or inhibited reflexly just as in normal conditions (Goltz). Vaso-dilatation is not, however, always an active effect, but sometimes only a cessation of normal tone; and there is one afferent nerve, the laction of which is always vaso-dilatation. This—the depressor, discovered by Ludwig and Cyon in 1866—plays a very important role in the regulation of the circulation. It is one of the afferent or sensory nerves of the heart, arising either in the heart itself, or, according to the researches of Koster mentioned above, at the base of the aorta. Stimulation of this nerve, arising normally from states of tension, inhibits the action of the vaso-motor centre and causes intense dilatation, more particularly of the abdominal system of blood-vessels. It is by the interdependence of the cardiac and vaso-motor nerve mechanisms, and the manifold ento- and epiperipherical stimuli by which they are influenced, that the heart is protected from undue strain; that great and prolonged varia-