Besides the nerve channels bearing impulses to the heart, others pass from the heart to the medulla, probably having their origin in the inner lining of the heart, which is the part most sensitive to stimulus.

These fibres appear to be of two kinds, one of which (in vagi) affects the cardio-inhibitory centre and diminishes the pulse rate; the other (depressor) affects the vaso-inhibitory centre and lowers the blood pressure. Increase of the intra-ventricular pressure stimulates both these sets of fibres, and- thus we see that over-filling of the heart from increase of blood pressure, etc., causes retardation of its beat, and an equilibrium is established between the general blood pressure and the force of the heart beat.