The examination of the development of the nervous system and of the muscular system in the vertebrata shows us their growing specialisation. The nervous cells are associated in systems more or less dependent the one upon the other; the muscular cells are accumulated into masses. This is the application of that law of the division of labour, the constant operation of which we observe in all the phenomena of life. The nervous cells are grouped together in a heap, in a nucleus, and send their prolongations to the periphery or to the organs. These prolongations are of two kinds: some transmit impressions towards the cell (dendrites prolongations), others transmit excitations proceeding from the cell (cylindraxes prolongations).[39] The centres themselves are hierarchised, so to speak, and are divided into two wide categories: the first destined to the functions of organic life, circulation, secretions, digestion, etc.; the second to those of the life of relation. These two categories include the sensitive cells and the motor cells; the one transmits to the other the stimulus born of excitations provoked by the internal or external centres. [39] _Dendrites_, nerves conducting the influx towards the centre of the cell. _Cylindraxes_, nerves conducting from the cell towards the periphery or towards another cell. In superior animals, at any rate in man, we observe that the activity of certain nervous centres is accompanied by a particular phenomenon, which is designated under the name of _personal_ consciousness. It is the notion we have that the phenomenon is perceived by us, that the movement executed is executed by us. Personal consciousness does not accompany all perceived phenomena, nor all executed movements. Certain given conditions of diverse orders appear necessary, for the consciousness to become aware of these phenomena. This conscious consciousness is translated by the connection of the impression or of the movement with a personality. This personality looks to us as though it were continuous. It is around it that our past impressions are grouped in the form of souvenirs. It is that which continues the ‘self.’ The consciousness I have just described is what I call the _personal consciousness_. The notion of personality which characterises it is not invariable, and is not necessary. It is not invariable, because the study of morbid psychology reveals to us that different personalities can succeed one another in the same individual, or even appear to be concomitant. This is notably the case with secondary personalities in hysteria or in epilepsy. It is not necessary, for diverse phenomena can be perceived and stored up in the memory without the personal consciousness being conscious thereof; in the same way, movements adapted to a certain purpose may be executed without the personal consciousness being warned thereof: such are notably the reflex and complicated movements, which custom has rendered automatic.