in the thermal and electrical condition of the whole organism, with corresponding changes in circulation, secretion, etc. So different are the consequences of two slightly different retinal impressions mingling their stimulations with the same mass of central substance! 182. The discharge is determined by two conditions: the state of tension, and the energy of the stimulation. _The state of tension is increased by every stimulation which falls short of a discharge_; that is to say, faint and frequent stimulation augments the excitability, whereas powerful stimulation exhausts it. When, therefore, one wave succeeds another in the same direction, it reaches a centre more disposed to discharge; or, as Cayrade expresses it, “a certain agitation of the cells is necessary for the manifestation of their property of reaction, in the same way that the concentric circles produced on the surface of water by a falling stone are more rapid and more numerous if a stone has already agitated the surface.” 183. So much for the tension. What has been called the energy of the stimulation is more complicated. It is not measurable as a simple physical process; we cannot say that a given quantity of any external force will determine a given discharge. It is mostly complicated by psychical processes, and these so modify the result that instead of the predicted discharge there is arrest, or discharge from another centre. Press a dog’s skin with increasing violence, and the effect increases from pleasurable to painful irritation; but whether the dog will cry out and bite, or cry out and struggle to escape, depends upon whether the pincher is a stranger or a friend. If you hurt a dog while removing a thorn from its foot it will cry out, but although the pain causes it to initiate a biting movement, by the time your hand is reached that movement will have been changed, and the dog will lick the hand which he knows is hurting him in the endeavor to relieve him of the thorn. The co-operation of the mind is here evident enough. A purely psychical process has interfered with the purely physiological process. And I shall hereafter endeavor to show that psychical processes analogous in kind though simpler in degree are really co-operant in actions of the spinal cord. The dog would be said to discriminate between the pain inflicted by a friend, and the same pain inflicted by a stranger. In other words, the sensitive mechanism would be differently determined in the direction of discharge, although the initial stimulation was the same in each case. If we admit that the resulting action is in each case the consequence of the particular group of elements co-operating, there will be no ground for denying that analogous _discrimination_ is manifested by the brainless animal, who also responds differently to different external stimuli, and differently to the same stimulus under different central conditions. The brainless frog croaks if its