=The Divisions of Psychology.=--So far as possible, then, we are to study states of consciousness in correlation with their probable neural conditions. Now the nervous system is well understood to-day to be nothing but a machine for receiving impressions and discharging reactions preservative to the individual and his kind--so much of physiology the reader will surely know. Anatomically, therefore, the nervous system falls into three main divisions, comprising-- 1) The fibres which carry currents in; 2) The organs of central redirection of them; and 3) The fibres which carry them out. Functionally, we have sensation, central reflection, and motion, to correspond to these anatomical divisions. In Psychology we may divide our work according to a similar scheme, and treat successively of three fundamental conscious processes and their conditions. The first will be Sensation; the second will be Cerebration or Intellection; the third will be the Tendency to Action. Much vagueness results from this division, but it has practical conveniences for such a book as this, and they may be allowed to prevail over whatever objections may be urged. CHAPTER II. SENSATION IN GENERAL. =Incoming nerve-currents are the only agents which normally affect the brain.= The human nerve-centres are surrounded by many dense wrappings of which the effect is to protect them from the direct action of the forces of the outer world. The hair, the thick skin of the scalp, the skull, and two membranes at least, one of them a tough one, surround the brain; and this organ moreover, like the spinal cord, is bathed by a serous fluid in which it floats suspended. Under these circumstances the only things that can _happen_ to the brain are: 1) The dullest and feeblest mechanical jars; 2) Changes in the quantity and quality of the blood-supply; and 3) Currents running in through the so-called afferent or centripetal nerves. The mechanical jars are usually ineffective; the effects of the blood-changes are usually transient; the nerve-currents, on the contrary, produce consequences of the most vital sort, both at the moment of their arrival, and later, through the invisible paths of escape which they plough in the substance of the organ and which, as we believe, remain as more or less permanent features of its structure, modifying its action throughout all future time.