to the parts of the body that they actuate, and their arrangement with respect to one another, are extraordinarily complicated, but some idea of them must be given, in order to render clear the ways in which the working of the nervous system becomes deranged in insanity and other disorders.

In passing the tall warehouses that line certain of our streets, we notice, projecting from below the roof, the arm of a crane, which is used for lifting heavy goods from the street to the upper floor. When a bale of goods has been raised by one of these cranes to the necessary height, the arm swings round on a pivot, and the bale is deposited on a projecting ledge. If we notice the man who guides this swinging movement of the crane, we observe that he holds in his hands two ropes, which pass over pulleys, one to the right, the other to the left, of the crane, and are then attached to the extremity of the arm. In guiding the crane round to the right, he pulls on the right-hand rope, and at the same time pays the left-hand rope out to the required extent. When the crane has moved far enough, the left rope is checked, and the arm arrested at the required point. The advantage gained by the simultaneous use of the two ropes is obvious. If only one rope were used at a time, the arm could, it is true, be swung round to that side; but the speed of its movement could not be regulated with any nicety, and the extent of the movement could not be regulated at all. The arm of the crane would either not move far enough, or it would swing round until it struck a violent blow against the side of the building.

The mechanism of the movements of the body is precisely similar to that of the movements of the crane. The vast majority of our movements are performed by the pull of the muscles on the bones, acting, like the ropes on the crane, on levers of the third order; and in the one case, as in the other, the lever, which is moved by the pull of a force acting in one direction, is steadied by the pull of a lesser force acting in the opposite direction. There is no instance in the body of a muscle without an antagonist muscle having

a precisely opposite action; and whenever a muscle begins. to contract and to pull upon its point of attachment, simultaneously its antagonist starts into action, and begins to pull in the opposite direction, so as to steady and smoothen and regulate and check the movement produced by the other. So that the physiological unit of movement is a pair of antagonistic muscles. Muscles are brought into action by the discharge of the grey matter of the central nervous system delivered through the nerve fibres, so that in order to produce such a duplex muscular action as is necessary, there must be some definite and appropriate

connection of a region of grey matter with the nerves running to the muscles.

Suppose B, B' to be bones, connected by a joint at J, and having attached to them two muscles, M and M', pulling in opposite directions. Let G be a region of grey matter, and N, N' nerves running from this region to the muscles. G is a reservoir of energy which is discharged by some impinging force whose origin we need not now inquire into. Upon the discharge of G a head of pressure is set up within it, and the energy presses upon all sides and seeks to escape. If the outlets N and N' are of equal calibre, the energy will pass out by them in equal amounts, the muscles will be equally stimulated, will contract with equal force,

and the bone B will not move, but will become rigidly braced up in its present position. If, however, the outlet into N is larger than that into N', more energy will escape by N than by N', the muscle M will be more strongly stimulated than M', will act more forcibly, the bone B will move in the direction of the arrow, and the limb will become more bent.

If the limb is required to move in the opposite direction, it is evident that there must be another region of grey matter, connected with the muscles by other channels, having a reverse proportion to that of N, N'. Thus there will be required a separate tract of grey matter for each separate movement; and each such tract of grey matter, so connected with muscles as to produce by its discharge a definite movement, is termed a nerve centre. Although each movement requires a separate centre, yet for each such centre it will not be necessary for a separate pair of channels (nerve fibres) to run to the muscles. It will be enough if the outlets from the centres into the nerves bear the requisite proportion, and this being secured, the outlets can empty their discharge severally into a single pair of channels common to all the centres actuating that pair of muscles. Thus, the movement of the bone B in the opposite direction to that of the arrow may be actuated by a centre G' whose outlet into N' is greater than its outlet into N.

Suppose now that it is required to bring into simultaneous action more than one pair of muscles, as indeed frequently happens in the execution of movements. In breathing, for instance, movements of the chest, abdomen and throat are executed simultaneously, and in forced breathing, as after exercise, or when there is some hindrance to the proper aeration of the blood, it becomes necessary to move simultaneously not only the chest, abdomen, and throat, but the mouth, nose, neck, and often the arms as well. We have seen that for each pair or group of antagonistic muscles operating a single movement, a separate nerve centre is necessary; so that for the simultaneous action of several

pairs of muscles, the simultaneous action of several nerve centres becomes necessary. How can the simultaneous discharge of several centres be effected? One obvious method suggests itself at once. If the centres, whose simultaneous discharge is needed, were all connected with another centre, then the discharge of this other common centre would set all the rest discharging simultaneously. Suppose A, B, and C to be three nerve centres actuating the muscles of the chest, abdomen, and throat respectively, by means of the nerves a a, b b, and c c, and suppose that from each of these centres there goes a cord or channel of communication to a common centre at D. Then the discharge of D will set going simultaneously the discharges of A, B, and C, and will produce simultaneous

A B a à b

D

C

action of the three sets of antago

nistic groups of muscles which these three centres represent. Again, the muscles of nose, neck, and mouth might be represented in three other centres, E, F, and bc c CC G, and these be grouped together by a central station at H, and then H and D connected with a still more comprehensive station at I; and then the discharge of I would bring about simultaneous action of the whole of the muscular apparatus employed in forced respiration

FIG. 10.

It is obvious that any number of muscles can be brought into simultaneous and duly proportioned action by a similar arrangement of duly proportioned channels proceeding from a single centre; and by such an apparatus even the movements of equilibration, which demand simultaneous and duly proportioned action of almost all the muscles of the body, can be actuated.

The majority of our acts do not, however, depend solely on the simultaneous action of muscles. In walking, for instance, while a number of muscles must act simultaneously to produce each movement of each leg, yet these movements

would be of little service if they were not timed to follow one another at proper intervals. So also in writing, in speaking, and in every form of handicraft, while each movement of arm and hand is actuated by the simultaneous pull of many muscles, the conspicuous factor in the success of the operation is the nicety with which each movement follows precisely in the nick of time upon the heels of its predecessor It is obvious that no single discharge from any one centre, however comprehensive in its control over the body, will account for a sequence of movements,—for the occurrence of a number of movements following one another in orderly succession. Since every movement requires the discharge of a separate centre, sequences of movements

must necessitate the discharge of many centres in succession; and each discharge must occur in its right place in the series, and at the moment at which the movement is required. However much the apparatus that we have already considered may be extended and developed, it can never assume a function of which it has not, as far as we have ascertained, acquired even the rudiment. To fit it for this new function a new factor is required.

Take as an instance the action of moving an object from one place to another. In this case the successive movements of stretching out the arm, grasping the object, moving and arresting the arm, and relinquishing the grasp, have to be made in due order. If the matter is considered, it will become apparent that to move the hand to the object, the