The disposition of the fibres in the brain is far too complex to be accurately followed. All that we can say is, that there are strands which connect one convolution with another, strands which connect one hemisphere with another, strands which connect cerebrum with cerebellum, and strands which connect the cerebrum with the lower ganglia. It is important to conceive this distinctly; for we shall hereafter see that the function of the Brain (by brain is here meant both Cerebrum and Cerebellum) is not that of _innervation_, but of _incitation and regulation_. To speak metaphorically, it is the coachman who holds in his hands the reins, and guides the team. One cardinal fact should arrest attention, namely, that not a single nerve in the body has its origin or centre of innervation in the cerebrum and cerebellum. The olfactory and optic nerves do indeed _seem_ to issue from the cerebrum; and are commonly described as cerebral nerves. But the facts of Development, minute Anatomy, and Experiment prove this to be inexact. Although I shall continue to speak of the olfactory and optic nerves in accordance with universal usage, not wishing to burden the reader with unnecessary innovations, I must at the outset express my opinion that these nerves cannot be brought under the same general type as the other sensory nerves. Embryology and Anatomy suggest that they have no more claim to the title than the _crura cerebri_. Of this hereafter. Setting these aside, no one now refuses to acknowledge that Cerebrum and Cerebellum, although centres of Incitation and Association, are not the centres of direct Innervation: the organic mechanism in all its _physiological_ processes will act independently of them (so far as such artificial distinctions are admissible at all). This does not throw a doubt on their _physiological_ functions, nor on their participation in the normal execution of physiological processes. 8. From this rapid survey two important points may be selected for special attention. First, the continuity of the neural axis throughout; secondly, the fundamental similarity of its _structure_, underlying great variations in its form and connections. This, which is the anatomical expression of the Unity of the nervous system, will become more evident after we have expounded what Embryology and Microscopic Anatomy teach. We may therefore digress here awhile to consider THE EARLY FORMS OF NERVE CENTRES. 9. In the outermost layer of the germinal membrane of the embryo a groove appears, which deepens as its sides grow upwards, and finally close over and form a canal. This canal is composed of cells all alike. Its foremost extremity soon bulges into three well-marked enlargements, which are then called the _primitive cerebral vesicles_. The cavities of these vesicles are continuous. Except in position and size, there are no discernible differences in these vesicles, which are known as the Fore-brain, Middle-brain, and Hind-brain.