14. The _Middle-brain_, or Second Vesicle, develops the _corpora quadrigemina_ from the roof of its cavity, and the _crura cerebri_ from its floor. The _Hind-brain_, or Third Vesicle, divides into two, like the First Vesicle; it buds off the hemispheres of the cerebellum; its cavity forms the fourth ventricle; its walls the _medulla oblongata_. 15. It thus appears that the primitive membrane forms into a canal, which enlarges at one part into three vesicles, and from these are developed the encephalic structures. The continuity of the walls and cavities of these vesicles is never obliterated throughout the subsequent changes. It is also traceable throughout the medulla spinalis. And microscopic investigation reveals that underneath all the morphological changes the walls of the whole cerebro-spinal axis are composed of similar elements on a similar plan.[86] 16. Two conclusions directly follow from this exposition:--first, that _since the structure of the great axis is everywhere similar, the properties must be similar_; secondly, that _since there is structural continuity, no one part can be called into activity without at the same time more or less exciting that of all the rest_. THE PERIPHERAL SYSTEM. 17. Following the analytical division, we now come to the Peripheral System of nerves and ganglia. The separation, I must often repeat, is purely artificial; but the artifice has conveniences. We separate in the same way the heart from veins and arteries, and the capillary circulation from the arterial. Each nerve has its direct connection with a particular centre, and indirectly with the whole system. It has its circumscribed territory, and individual office. Except in a few cases of anastomosis, the action of one nerve does not involve that of another: only one muscle or one group of muscles is moved, without exciting motion in a neighbor. It is through the centres that these individual territories are united; and a wave of excitation always passes throughout the central substance. Thus the centres are not simply organs of association, consequently of regulation, but are the nexus whereby the diversity of the actions is integrated into the unity of consensus. 18. Nothing further need at present be stated respecting the nerves; but it is needful to give precision to the ideas of GANGLIA AND CENTRES,