REFLEX ACTIONS. A large part of the beautiful adaptation and coardination of the different parts of our bodies is effected by the reflex activities of the nervous system entirely outside our con scious knowledge. The way in which these re flexes are produced is illustrated best by the simple reflex movements that can he obtained, from a headless frog. If in such an animal, with its spinal cord intact, one of the toes is gently pinched, the leg will at once be drawn in toward the body. if the skin is stimulated by bits of paper moistened with dilute acetic acid, the leg of the corresponding side will be raised and the piece of paper will be wiped off by a neat move ment involving the coordinate activity of a num ber of muscles. What happens in these cases is that the stimulus applied to the skin irritates the sensory nerve fibres and sends in a stream of nerve impulses to the spinal cord. ' These im pulses are conveyed to the dendrites of certain motor nerve cells and arouse in them fresh im pulses which are conducted outward through the efferent nerve fibres to the muscles. The original stimulus, or, more accurately, the nerve impulses to which it gives origin are thus, as it were, re flected in the nerve centres and sent to the peripheral tissues, whence the name of reflex action. Every such reflex involves the activity of at least two groups of nerve cells, one con nected with the sensory fibres stimulated and one connected with the efferent fibres going to the muscles. Such reflexes may occur in all parts of the spinal cord and brain. A common reflex in ourselves, for instance, is the winking of the eye when its sensory surfaces are touched. As a rule we limit the term reflex action to those cases in which the element of consciousness is not involved. As a matter of fact,, however, all of our conscious processes and mental activities are effected by a similar action of one nerve cell or unit on another, the mechanism in action being essentially similar to, although more complex than, the simple reflexes of the spinal cord.
In a mammal as well as in a frog it can be shown that if a part of the spinal cord is severed from its connections with the brain, under such con ditions that death does not result at once, the part of the cord below the injury will suffice to effect complex movements upon appropriate stimulation of the skin. To the uninformed such movements usually suggest consciousness, lout the evidence of physiology proves conclusively that they are entirely uneonseious reflexes.
It should he added that the outgoing impulse in reflex stimulation of the cord or brain may proceed through nerve fibres connected with other tissues such as the heart-muscle, the plain muscle of the viscera or the glands, and the reflex effect, instead of exhibiting itself as a movement of the limbs, may take the form of a secretion, of a change in the heart beat, or of a constriction or dilatation of the blood-vessels. Owing to the complexity of the connections among the numerous nerve cells in the spinal cord and brain, it is evident that reflex effects may be widespread and very intricate. Certain definite paths or connections are inherited or become acquired during life from repeated use, so that the passage of a sensory nerve impulse that reaches the central nervous system is not hap-hazard, but for the most part along prede termined paths. The wonderful instincts of the lower animals may be regarded as complicated reflexes. and the unconscious regulation of the different parts of our body, especially the activity of *lte internal organs or viscera, is controlled in this way by the nervous system.