Page:Life in Motion.djvu/215

Rh may be made without the person or animal being conscious of making them.

Now, whether movements are voluntary or involuntary, they always require a nervous mechanism having the same structural type, although it may be more or less complicated by the necessities of the act to be performed. The simplest form of this mechanism is what we term in physiology a reflex mechanism. It consists of a centre, a sensory or afferent nerve, carrying impressions to the centre, and a motor or efferent nerve, carrying impressions from the centre to something in the circumference or periphery of the body. Thus, if we pinch the toe of a decapitated frog, it draws the leg away. The pinching irritates the sensory nerve, something, as you now know, travels along it to the nerve-centre, which, in this case, is in the frog's spinal marrow, and from the spinal marrow, after the lapse of a little time (the latent period in the marrow) a new impulse starts outwards along motor nerves to the muscles, reaches these, and causes them to contract. There are many varieties of these reflex acts. We may be quite unconscious of them, or we may feel the stimulus,