A system of practical medicine. By American authors. Vol. 5

by Unknown

(f) Doubtless the internal muscular organs and the blood-vessels are frequently the seat of inco-ordinate or quasi-ataxic movements, but our present knowledge of these conditions amounts to very little.

It may be permissible to consider vertigo under the general head of inco-ordination, because it usually finds a motor expression, either actual or subjective. Subjective vertigo consists in a sense of whirling or horizontal movement which is clearly referred to the brain by the patient. The sensation is variously expressed: in some the head seems to whirl around; in others external objects seem to whirl about the patient horizontally or vertically; in others still there is a sensation of falling in a given direction or of dropping into a hole. Dizziness is a minor degree of subjective motion conjoined with more or less confusion and other paræsthesiæ in the head. Static vertigo is that form in which there is actual loss of equilibrium, or falling. It is observed more especially in diseases of the internal auditory apparatus, and it may be produced artificially by galvanization of the brain. If the electrodes are placed exactly on the median line, and the current passed fronto-caudad through the brain, no loss of equilibrium or vertigo is produced. If, however, the current is passed transversely through the brain, there is produced at the moment of the closure and opening of the circuit a distinct tendency to fall or an actual fall to one side: in closing the current the fall is always away from the negative pole or cathode. The greatest amount of disturbance is produced when the electrodes are placed on the mastoid processes or near them. Whether the disturbance of equilibrium is caused by a change in electrotonus in the hemispheres, or by a change in the vascular supply of the hemispheres (in one anæmia, in the other relative hyperæmia), is uncertain. A third explanation is equally plausible—viz. that the vertigo is caused by unequal excitation of the internal acoustic organs and the two halves of the cerebellum. This view would be supported by the fact that in animals and man a lesion of the acoustic nerve and of the processus ad pontem on one side produces strong rotary movements about the long axis of the body.

ABNORMAL REFLEX MOVEMENTS.—Some reference has already been made to absence or exaggeration of reflexes in the preceding paragraphs, but the importance of these symptoms demands that they should be separately considered.

A reflex action, in its simplest conception, is a movement (muscular, vascular, or psycho-motor) or a secretion which is the result of the transformation of a centripetal impression into a centrifugal impulse. The apparatus required for the accomplishment of the reflex action consists essentially of a sensitive surface of skin or mucous membrane, of tendinous or other deep structure, to receive the impression; a sensory afferent nerve to convey it; a nervous centre of the simplest structure (one or two ganglion cells) to receive and transform it; a motor or efferent nerve to transmit the resulting impulse to a common muscle, to the vascular wall of a vessel or viscus, or to a gland. These various terminal organs execute their normal functions in response to the centrifugal impulse, and thus give external or tangible evidence that the reflex action or reflex is completed. This mechanism, the reflex arc, is illustrated by the diagram (Fig. 1), which will also serve for the explanation of many of the morbid reflex states.

FIG. 1.

Diagram illustrating the Arc for Reflex Action. The centripetal and centrifugal paths, the receptive and terminal organs of the arc, are shown connected with the spinal centre, which is a portion of central gray matter containing ganglion cells. The inhibitory cerebral influence is constantly exerted in health to moderate reflexes.

In the healthy animal body many important functions are performed under the law of reflex action; for example, many organic or secretory functions, the movements of hollow viscera, vascular movements, muscular tonus, defensive and expressive muscular movements. In the process of education many acts which at first must be learned by oft-repeated efforts, such as walking, playing upon musical instruments, etc., later become habitual and automatic, and are then largely executed by reflex action unconsciously, co-ordinated by the spinal (?) apparatus. Much of our intellectual activity is governed by the same law operating between the psycho-sensory and the psycho-motor apparatus.

Reflex actions are only to a moderate extent under the control of volition—indeed, most of them are subconscious—but a strong restraining action is unconsciously exerted by the cerebrum (vide Fig. 1) upon the spinal reflex centres. This is the so-called inhibitory cerebral influence. The existence of such an influence is shown by experiments upon animals, and by human cases in which, the cerebral activity being reduced or abolished, we observe a great increase in the frequency and extent of reflex movements, particularly in the common muscular apparatus.