§ 1. Physiological Antecedents and Consequents of Mental Process.—It is an old saying that the body is the organ of the mind,—the instrument through which it works. This account of the matter is evidently true so far as regards the peripheral organs of sensation and motion. We cannot see without eyes, or hear without ears; we cannot move without muscles. But without sensation and motion the process of consciousness would be impossible. We can give no satisfactory account of the mental life without reference to the construction of the organs of sense and the impressions they receive from external agencies, or without reference to the mechanism of movement by which we act on the external world and change our spatial relations to surrounding objects. There is practically no difficulty in determining the nature of the relation between body and mind when we have in view on the bodily side only the peripheral instruments of sensation and motion. For all psychological purposes this must be regarded as a relation of interaction. Impressions on eye or ear produce modifications of consciousness, and conscious volitions produce muscular contractions. Serious difficulties arise only when we push our inquiry further back, and examine the relation of nervous process to conscious process.

* This is not an easy subject. The student is recommended to do his best to understand this chapter on the first reading, but should certainly make a point of returning to it after having read. the book through.

Muscular contraction follows change in consciousness only when the muscle is excited by an impulse which has its origin in a disturbance of the grey matter of the nervous system. Similarly, impressions on the organs of sense produce sensations only when they set up an impulse which is transmitted to the brain. In this process some parts of the nervous system may be regarded as intermediate links. Consciousness is not immediately connected with the occurrences which take place in them. These occurrences are either antecedent conditions of the nervous changes which are directly connected with consciousness, or consequences ensuing from nervous change that is directly connected with consciousness. They thus constitute intermediate stages between change in consciousness and change in the peripheral organs, and inversely. The nervous mechanisms which fulfil this mediating function may be regarded, like the muscles or the senses, as organs in the service of mental process. The relation is one of interaction. Conscious volition produces change in the nervous :mechanism, and change in the nervous mechanism, set up in the first instance by impressions on the organs of sense, produces changes experienced in consciousness as sensations. But the case is essentially different for those nervous processes which are connected with consciousness immediately, without the intervention of any other material occurrences. This unmediated connexion of neural and conscious occurrences is found mainly, if not exclusively, in the cerebral cortex, which is the highest part of the brain. "Viewed broadly, the brain is a mass of white matter, with nuclei of grey matter deeply imbedded in it, and with a sheet of grey matter, about onefifth of a square meter in area and between two and three mm. thick, covering the folds, fissures, and convolutions of its surface."* This overlying sheet of grey matter is the rind or cortex of the brain, and is in immediate connexion with conscious process. For our present purposes, we may without serious inaccuracy regard all processes taking place in other parts of the nervous system as connected with consciousness only so far as they are causes or effects of processes in the cortex. Before coming to the vital question of the relation between cortical process and conscious process, it will be convenient to give some account of those parts of the nervous structure which lie beneath the cortex. +