Pear-shaped cells are set round a felt-work of nerve-fibrils (neuropil). A neuro-sensory cell is shown with one fibre directed peripherally, branching on the surface; and one directed centrally, ramifying in the neuropil. Several very slender fibrils from the neuropil pass up the stalk of each ganglion-cell. They join a network near its surface. This net is connected by radiating fibrils with a coarser net which surrounds the nucleus. From the central net a relatively stout fibril passes to muscle-fibres.] By various methods it may be shown that dendrites, cell-body, and axon contain fibrils (Fig. 22). These neuro-fibrillæ lie parallel to one another in the axon. Where it divides they are distributed amongst its branches. Possibly they also branch. In the neurones of Malapterurus, already referred to, this would appear to be inevitable. The discovery of neuro-fibrillæ seemed to carry us a step nearer to a comprehension of the physics of nervous conduction. They clearly indicate that particles of the substance of a nerve-fibre are oriented in the direction in which impulses pass. It is a structural differentiation similar to the fibrillation of muscle, and probably of the same order—a response to the same demand. But when we examine the arrangement of the fibrils in a cell-body and its dendrites, the appearances which we discover serve to perplex us. They complicate instead of simplifying our mental picture of the conduction of nervous impulses. The coarsest and most distinct neuro-fibrillæ are to be found in annelids, the ganglion-cells of a leech, for example, affording excellent preparations. These cells are pear-shaped, with a single stalk. As is usual in invertebrate animals, they do not exhibit separate dendrites and axon, but dendrites and axon pass out from the cell in the common stalk. The bodies of the cells are set round a felted mass of nerve-filaments, into which their stalks break up. Just beneath the surface of the stalk of one of these cells two or three very fine neuro-fibrillæ are to be seen. A single, much coarser fibril occupies its axis. The fine fibrils join a network at the periphery of the cell-body. The thick fibril is connected with a coarser network which surrounds the nucleus. Radiating threads unite the finer with the coarser net. It has been suggested that afferent impulses ascend the fine fibrils, pass from the finer to the coarser net, and take their exit by the thick fibril, which can be traced into a motor nerve. Such a transit could not, so far as one can imagine, have any effect upon the distribution of the impulses which pass through the neurone; besides, there are reasons for believing that the course taken by impulses which are delivered to the ganglion by sensory nerves is determined by the felt-work in its centre, the neuropil. It is probable that during their passage through the cell-body impulses