Most people are familiar with the roaring sound which may often be heard in their ears during strong contraction of the jaw muscles when the external meatus of the ear is blocked at its outer end. We have, however, been able to hear in quiet surroundings a succession of discrete clicks or pops occurring during much slighter degrees of muscular contraction. These clicks occur with a fairly regular rhythm whose frequency varies with the strength of the cbntraction. We were struck by the resemblance of these rhythms to a loudspeaker reproduction of the action currents from single motor units, and it seemed probable that the sounds were the mechanical counterparts of the muscle action currents. This paper deals with the relation between these two phenomena. APPARATUS AND METHODSAll the experiments to be described were carried out on intact human subjects.Whenever muscle action potentials were recorded, they were picked up through the skin by means of bipolar electrodes. The electrodes consisted of a pair of parallel wires of enamelled constantan between 0-06 and 0-12 mm. in diameter. The ends of the wires were cut square, and the flat uninsulated surfaces were applied to the skin, with the axis between their centres in line with the main axis of the muscle fibres. The separation of the electrodes varied, in different experiments, between 0.1 and 1-5 mm. The subject was earthed through a silver plate which he held in his mouth, and this procedure reduced interference from the mains enough to make the use of a shielded room unnecessary.When investigating the mechanical disturbances underlying the clicking sounds, two types of electro-mechanical instrument were used. The sounds were recorded by means of a crystal microphone. Alternatively, a stiff piezo-electric system, in direct contact with the skin, was used to record mechanical movements of the underlying muscle, without the intervention of a column of air. In order to record simultaneously the electrical action currents and the mechanical effects, a pair of wire electrodes, like those just described, was incorporated in each of these instruments.The potentials from the wire electrodes and from the microphone or mechanical device were led separately to two resistance-capacity coupled push-pull amplifiers having degeneration of in-phase signals. The amplifier used for action potentials has an overall time constant of about 10 msec.,

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The dorsal column nuclei (nucleus cuneatus and nucleus gracilis), receiving many primary afferent fibres from the dorsal columns and sending many secondary fibres into the medial lemniscus, might be expected to be interesting in that they could contribute to the central analysis of sensory information. These nuclei appear to have a high factor of safety in transmission, as judged by the work of Therman (1941), who studied their function as 'relays'. More recently, it has been suggested that they contain paths that are not purely monosynaptic and that their organization is therefore more complex than that of simple relays; and limited spatial facilitation and inhibition of single neurones have been described (Amassian & de Vito, 1957).We have approached this problem from the point of view of the way or ways in which impulses set up by combinations of tactile stimuli are handled by these nuclei, bearing in mind that such phenomena as mutual spatial inhibition have been seen in other primary sensory nuclei-in the visual and auditory pathways, for instance-and are likely to be relevant in sensory discrimination. We have therefore been concerned particularly with the cutaneous receptive areas of the cells and with spatial interactions, whether facilitatory or inhibitory, upon these cells. We have largely confined ourselves to those cells in nucleus gracilis which responded to tactile stimulation of the hind foot. A preliminary communication on this subject has already been made (Gordon & Paine, 1959). METHODSPreparation and fixation of animaws. Cats were used for all the experiments. The great majority were anaesthetized with sodium pentobarbitone, the initial dose given intraperitoneally (38 mg/kg) and subsequent small doses of about 10 mg given by intravenous cannula. Anaesthesia was maintained at a Jevel sufficient to prevent any spontaneous movement of the animal or movement in response to stimulation. A few animals were decerebrated under ether by blunt section at intercollicular level; and at some stage after recovery from the anaesthetic it was always necessary to prevent movement in these animals, e.g. by giving 1 mg doses of decamethonium iodide intravenously, respiration being maintained with a pump.

It can be assumed that the dorsal column nuclei and medial lemniscus provide an orderly system of topographical projections through which information from the whole body surface reaches the thalamus. A particular part of the body is 'represented', throughout the long axis of nucleus gracilis, by a rostral section whose cells have large receptive areas, a middle section with small receptive areas and considerable mutual inhibition, and a caudal section with intermediate receptive areas (Gordon & Paine, 1960). These sections differ functionally; and we felt that an investigation of their projections, in the lemniscus and elsewhere, would help one to understand the meaning of this arrangement. It would also help to show in what form the thalamus receives its information.

THE human M. orbicularis oculi takes part in many patterns of movement. Some of these, like the-corneal reflex, the narrowing of the palpebral orifice in bright light, or the adjustments which accompany upward or downward rotation. of the eyeball, are connected in a comprehensible way with the eye itself. The muscle also plays its part in complex movements of the face, such as smiling. A third and particularly interesting type of movement is that of unconscious blinking, the pattern of which is undoubtedly related to visual perception (Hall, 1945), but which nevertheless occurs with about the normal frequency both in the blind, and in subjects in whom the cornea has been made anaesthetic (Ponder and Kennedy, 1928).It is known from direct observation that there is some functional subdivision between the different anatomical regions of the orbicularis (Gad, 1883; Whitnall, 1921), and an attempt is made in this communication to amplify these earlier descriptions by means of electromyographic observations. It was found in the course of another investigation (Gordon and Holbourn, 1948) that the action potentials of single motor units could easily be recorded by means of wire electrodes placed upon the surface of the eyelids, and in this way one may get definite information about the activity of the motor neurones supplying this muscle. The opportunity has also been taken to investigate the activity of M. levator palpebrae superioris in the monkey.METHODS HUMAN ExPERIMENTs.-The great majority of the observations were made upon my own eyelids. After some preliminary work, attention was limited to my left eye, of which a photograph was taken and a millimetre grid superimposed upon the photograph. The points from which observations had been made were recorded by reference to this grid.The electrodes used for recording the action potentials of the orbicularis through the skin were described in an earlier paper (Gordon