the same resistance is offered to the passage of impulses along the fibrillæ where they lie within the stout branches of the cell-processes or within the body of the cell. It is easy to make a pictorial representation of such a mechanism. Imagine a model of the stem of a tree made by binding together a large number of wires; its branches as containing small groups of wires; the ultimate twigs as separate wires. Carry wires from the roots of one tree to the branches of another. Trees so constructed might be taken as representing nerve-cells. We have not as yet succeeded in demonstrating the isolated neuro-fibrillæ as they pass over from the end-twigs of a nerve-fibre to the end-twigs of a nerve-cell branch, but we have abundant reason for believing that they do so pass, and that the resistance to the passage of a nerve-impulse is interposed in this neutral or junctional zone. This resistance has to be overcome. It is overcome by the summation of impulses. All nerve-impulses are vibratory. The first vibrations may fail to get through; but if the vibrations continue, they exert a cumulative effect. After a time they overcome the resistance; sensory impulses flow through the centre into motor channels. In this way we endeavour to explain the rhythmic discharge through the respiratory and other centres. It has not been found possible to determine the source of all the afferent impulses which reach the centre. Respiration continues after all accessible nerves have been cut, including even the posterior roots of the cervical nerves. Probably it is a mistake to look for definite afferent channels in the medulla and the rest of the brain. All parts of the body need aerated blood. From all parts, including nerve-tissue itself, arises the demand for respiration. Possibly nerve-centres have the power, as it were, of storing impulses, and discharging them after the stream of fresh arrivals has ceased to flow. They may acquire a habit.