Page:Popular Science Monthly Volume 75.djvu/60

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HE physiological unit in the operations of the nervous system is the reflex. Broadly understood, this consists of the chain of consequences that begins with the reception of a stimulus on the surface of the animal and, leading through the central nervous organs, ends in the excitation of a reaction by some such organ as a muscle. The term reflex is made to apply nowadays to nervous operations involving conscious states as well as to those that are carried out unconsciously. In its greatest simplicity the conventional reflex involves at least two nervous cells or neurones and some form of reacting organ

 (modified from Retzlus). cm, circular muscle; ep, epidermis; lm, longitudinal muscle; mc, motor cell-body; mf, motor nerve-fiber; sc, sensory cell-body; sf, sensory nerve-fiber; vg, ventral ganglion.

such as a muscle-fiber. The first neurone, as exemplified in the nervous structure of such an animal as the earthworm, is often the body of a sense-cell on the surface of the animal and the sensory nerve-fiber to which this cell body gives rise and which leads to the central nervous organ. The second neurone is a nerve-cell whose body lies within the central nervous organ and whose process, a motor nerve-fiber, extends from the central organ to the muscle-cells which it