RESPIRATORY MOVEMENTS.

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much as the two acts are opposed to each other and must take place by turns. The commencing stimulus in this case requires to be a muscular stimulus, an effect arising out of the action of muscles, and not the irritation or compression of a surface. The completed action, the fully accomplished contraction, of one set of muscles must affect the centre so as to commence the action of the other set. There would require to be distributed to the same muscles two classes of nerves, one for conveying influence inward to the medulla oblongata, the other for receiving the motor stimulus arising there; and the circles would require to be so organized, that the nerves conveying influence inward from the muscles of inspiration should pass to those portions of the grey substance of the medulla that send out motor nerves to supply the muscles of expiration; and, conversely, the ingoing nerves from the last named set should be in relation with the motor nerves belonging to the other set, those of inspiration; while the stimulus proper to these incarrying nerves, the influence that they are adapted to convey, should be that arising from the full and complete contraction of their respective muscles. Such is the arrangement that we are obliged to assume or suppose, in order to account for the double action. It is important for us to recognise this mode of mechanism in this its simplest example, as we shall have reason for believing that the same mode prevails extensively throughout the bodily and mental constitution.*

There are certain special movements occasionally executed by the respiratory apparatus likewise belonging to the automatic or reflex class. Coughing is one of those. The stimulus in this case, however, is a true surface stimulus; the contact of foreign matter with the interior wall of the bronchial tubes is the cause of the spasmodic movement. The irritation of the bronchial surface originates a stimulus propagated to that

When the sensory nerve distributed to the surface of the lungs is cut through, the breathing action is weakened, showing that a certain amount of stimulus is derived from the action going on throughout the surface. If, farther, the brain is paralyzed by any poison, the respiration is still more enfeebled, leading us to infer that the brain contributes to the breathing activity. What remains after deducting those two aids requires the supposition made in the text to account for it.

E

part of the medulla oblongata that sustains the action of the lungs. The consequence is a momentary increase of the expiratory force, the glottis being closed and opened suddenly, so as to amount to an explosion, or a shot, which propels the material out of the tube. The action of the lungs is peculiarly liable to be raised to this explosive pitch, owing, we may suppose, to the readiness of the medulla oblongata to give off sudden discharges of central energy. Sneezing differs from coughing in the circumstance that the seat of irritation is the nose. The course of the explosive current is on that account directed through the nostrils.

Among the reflex influences exerted upon the lungs through spinal intervention we are to include the stimulus of cold suddenly applied; which stimulus acting on the surface of the lungs in the shape of cold air, or on any part of the skin, as in the cold bath, reinforces the breathing energy. This influence is accompanied with a very keen sensation, but the instantaneous reaction that increases the movement of the lungs is believed to be entirely independent of sensation or will, and is attributed with appearance of reason to the medulla oblongata. The hysteric laugh, which is one of the effects of cold, points to the mediation of the medulla even in the effects arising from the sensation.

(3.) The winking of the eyes is essentially automatic. Although not entirely withdrawn from the control of the mental centres, this movement of the eyelids usually goes on independent of these centres. The stimulus to the movement is a surface stimulus, apparently due to the liquid that washes the eye, and in so doing comes in contact with the inner surface of the upper eye-lid. When an action takes place on this inner surface an influence is transmitted inwards to some centre, and there is reflected a stimulus to the muscle that closes the lids. One may try the experiment by touching the edge or inner surface of the upper eye-lid with anything solid; instantly there is produced a spasmodic flutter of the eye-lid, or

* Probably the corpora quadrigemina.

logists asminet ILE I am and wieved in an a peculiar innares T. & tone, or wancing fie muss near the that a mousie erly ruset vie me ama sain Even in the perden repas if seen a ran vigour of cmnatan deng na de mes of bury The force of eman reset ane moment É wakening, and sme vien an eft is be made, but at no time is the rearamo na, the is never dangle like a loosely constructed in mai afer the amal is dead. there is a certain amount of the permanent contractile

part of the medulla oblongata that sustains the action of the lungs. The consequence is a momentary increase of the expiratory force, the glottis being closed and opened suddenly, so as to amount to an explosion, or a shot, which propels the material out of the tube. The action of the lungs is peculiarly liable to be raised to this explosive pitch, owing, we may suppose, to the readiness of the medulla oblongata to give off sudden discharges of central energy. Sneezing differs from coughing in the circumstance that the seat of irritation is the nose. The course of the explosive current is on that account directed through the nostrils.

Among the reflex influences exerted upon the lungs through spinal intervention we are to include the stimulus of cold suddenly applied; which stimulus acting on the surface of the lungs in the shape of cold air, or on any part of the skin, as in the cold bath, reinforces the breathing energy. This influence is accompanied with a very keen sensation, but the instantaneous reaction that increases the movement of the lungs is believed to be entirely independent of sensation or will, and is attributed with appearance of reason to the medulla oblongata. The hysteric laugh, which is one of the effects of cold, points to the mediation of the medulla even in the effects arising from the sensation.

(3.) The winking of the eyes is essentially automatic. Although not entirely withdrawn from the control of the mental centres, this movement of the eyelids usually goes on independent of these centres. The stimulus to the movement is a surface stimulus, apparently due to the liquid that washes the eye, and in so doing comes in contact with the inner surface of the upper eye-lid. When an action takes place on this inner surface an influence is transmitted inwards to some centre, and there is reflected a stimulus to the muscle that closes the lids. One may try the experiment by touching the edge or inner surface of the upper eye-lid with anything solid; instantly there is produced a spasmodic flutter of the eye-lid, or

* Probably the corpora quadrigemina.

MUSCULAR TONICITY.

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a very rapid succession of winks. The reflex act goes to closing the eye, and the opening is effected by the muscles that keep the eye open during the ordinary waking state. In sleep, the winking muscle is unresisted and keeps the eyes shut. The activity of this muscle, (called the orbicularis) is shown by this fact to be purely reflex, for no voluntary movement is sustained when we are asleep.

(4) It is to be considered how far the muscular movements generally, the locomotive and other actions of the body at large, are sustained by the spinal cord. We have already seen that convulsive movements of the limbs can be excited in a decapitated animal; and the question arises, does the cord keep up any of the regulated motions of the animal body, such as walking, running, flying, swimming, &c. The answer is that the cord does not seem capable of maintaining these motions. For, although there exists an innate power of performing them in many cases, other centres besides the cord are essential to their performance. In fact, the cerebellum is looked upon as the centre of the higher order of combined actions, notwithstanding that one pair of alternating movements, as in breathing, can be kept up by the cord alone. The locomotive movements of animals immediately after being decapitated are not a proof of the power of the cord acting by itself, inasmuch as these may be owing to the yet unexhausted stimulus of the brain, or they may be actions induced upon the cord, in consequence of habit.

There is one instance of muscular action by most physiologists ascribed to the spinal cord, and believed to have a peculiar interest in this point of view; that is, the tension, tone, or tonicity of the muscles. By this is meant the fact that a muscle is never wholly relaxed while the animal is alive. Even in the perfect repose of sleep there is yet a certain vigour of contraction inhering in all the muscles of the body. The force of contraction is increased at the moment of wakening, and still more when an effort is to be made, but at no time is the relaxation total; the limbs never dangle like a loosely constructed doll until after the animal is dead. Now there is a certain amount of this permanent contractile force