regarded as separate systems quite apart from each other, but as two sides of one system, which are essentially and closely related to each other. There is a provision for combined action of the two sets, so that an impulse communicated along a sensory nerve or set of nerves, may pass over to the motor system and terminate in muscular activity. This is most simply illustrated by the circumstance that the nerves of sensibility become instruments of pain, when a sévere shock or blow is given, or some injury is inflicted. Suffering becomes a signal of risk and instantly the injured part shrinks or starts away from the source of suffering. This is a phase of sensori-motor activity illustrating a law which has a wide range of application in animal life. This sketch of the arrangements and functions of the two sides of the nerve system though traced in view of its application to human nature, will suffice to indicate the general plan in accordance with which sensibility and muscular activity are provided for in the animal kingdom generally. The ramification of the nerve lines will in each case be according to the simplicity or complexity of structure belonging to the animal; but the

provisions for sensitiveness to touch, and power of movement are in all cases the same. Fish, bird, and quadruped are alike sensitive to touch, and they are alike capable of movement, though the mechanical contrivances by which locomotion is secured vary greatly; but a double distribution of nerve fibres in all cases provides for these two characteristics of animal life.

From this, we advance to the nerve centre, -the brain, to which the nerves of sensibility run up, and from which the nerves of motion come forth. Here also there is identity in the nature of the organ, while there is variety in its size, with more or less complicated plans of arrangement, according to the extent of the nerve system of which it is the central organ. Still keeping to the human body for illustration, we may find in the most. complex organism known to us illustration of what holds good in the main so far as essential structure is concerned.

The brain is made up of two entirely distinct substances. In the interior of the organ, and altogether concealed from view when a drawing of it is made, or the organ itself is exposed to observation, is a white mass con

sisting of a multitude of fibres. These are simply crowds of nerve lines gathered together, led up from the extremities and trunk, or provided for intercommunication with the several parts of this central organ. Gathered all round about this, and constituting the external mass, on the summit, sides, and base of the brain, is a completely distinct substance known as the grey matter, folded up in wavings, twistings, or convolutions, enclosing myriads of cells from which nerve energy is discharged. These cells differ considerably in form and size, suggesting the possibility of distinct functions being assigned to cells of different structure, some being smaller and less intimately connected with those around, others so much larger and more important as to have suggested the name of pyramidal cells, and also having lines of connection between themselves and other parts much more numerous than in the case of the smaller cells. Every cell has a nucleus or central point, which is the centre of vitality, while the fibres which they send out, varying in number from one to four or five, establish connection between cells, or pass into the nerves proper. These cells are packed together in a soft glu

tinous substance, in the outer layer of which they are fewer in number; approaching the interior, they become more numerous; and they are both more abundant, larger in size, and more distinguished by the number of their protoplasmic fibres as they lie nearer to the mass of nerve fibres. In this crowd of nerve cells are the stores of nerve energy supplied to the nerve system, with every exercise of which molecular changes in the brain are believed to take place. On this account there must be regular and ample supply of nourishment for the brain, for which such provision has been made that, according to Haller's computation, one fifth part of the whole blood supply goes to the brain.

Regarded as the great central organ, the brain is divided into two halves or hemispheres, from each one of which goes forth supply of nerve fibres and nerve energy for the opposite side of the body. Its greatest depth is in the central part, the front and back being rounded down, the frontal region being, however, considerably more massive than the rear. Besides this great central body, there are several dependent subordinate bodies, * Protoplasm, see Appendix VII.