. The chordates. Chordata. Integrative Systems 141 111. I EPINEURIUM NODE OF RANVIER P= 111 NEURILEMMA MYELIN SHEATH NEURITE(AXON) -ENDONEURIUM NUCLEUS OF NEURILEMMAS. A LONGIT SECTION SECTION Fig. 132. A portion of (A) longitudinal and (B) cross section of a nerve prepared by the vom Rath method. A nerve is a bundle of axons (neurites) covered by an epineurium. Each axon is surrounded by an inner myelin (fatty) sheath and an outer cellular neurilemma sheath. (Courtesy, Neal and Rand: "Chordate Anat- omy," Philadelphia, The Blakiston Company.) nervous organ is designated as a
. The chordates. Chordata. Integrative Systems 141 111. I EPINEURIUM NODE OF RANVIER P= 111 NEURILEMMA MYELIN SHEATH NEURITE(AXON) -ENDONEURIUM NUCLEUS OF NEURILEMMAS. A LONGIT SECTION SECTION Fig. 132. A portion of (A) longitudinal and (B) cross section of a nerve prepared by the vom Rath method. A nerve is a bundle of axons (neurites) covered by an epineurium. Each axon is surrounded by an inner myelin (fatty) sheath and an outer cellular neurilemma sheath. (Courtesy, Neal and Rand: "Chordate Anat- omy," Philadelphia, The Blakiston Company.) nervous organ is designated as afferent or sensory; one which trans- mits away from a central organ and toward a reacting organ is efferent or motor. The reacting organ, usually a muscle or gland, is called an effector. In the case of a gland, the efferent fibers may be related to the secreting cells themselves, controlling their secretory activity, or to muscle-fibers in the wall of the gland, controlling the discharge from the gland. A nervous reaction of the simple type known as a reflex is initiated by stimulation of a receptor. A nervous impulse produced by effects of the stimulation is transmitted from the receptor along a conductor, which may be a nerve-fiber belonging to the receptor cell (as in the olfactory organ), or a fiber of another neuron (as in the ear). From this conductor, the impulse is relayed over to another neuron which is related to the effector. But few reactions are as simple as this. Most reflexes are complicated by the fact that additional neurons are inter- polated between the two conductors of the simple "reflex arc" just described (Fig. 133). By this means, a simple and sharply localized stimulus may bring into action a complex system of effectors, as when a pin-prick causes a jerking of the entire arm. It is these interpolated neurons—association and correlation neurons—which make up a large part of the bulk of the central nervous organs. The physical nature of the
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