. The chordates. Chordata. 142 Basic Structure of Vertebrates SPINAL CORD. A. DIAGRAM OF A REFLEX ARC Fig. 133. Diagram of a nervous arc. (A) Three neurons—afferent, intercalary, and efferent—are shown in their relations to one another and to the skin and muscle. The intercalary neuron is located in the gray matter of the spinal cord. (B) An enlarged section of a nerve-fiber. (Courtesy, Neal and Band: "Chordate ," Philadelphia, The Blakiston Company.) mated so that the nervous impulse passes from one fibril to another in a manner analogous to that in which an electric current
. The chordates. Chordata. 142 Basic Structure of Vertebrates SPINAL CORD. A. DIAGRAM OF A REFLEX ARC Fig. 133. Diagram of a nervous arc. (A) Three neurons—afferent, intercalary, and efferent—are shown in their relations to one another and to the skin and muscle. The intercalary neuron is located in the gray matter of the spinal cord. (B) An enlarged section of a nerve-fiber. (Courtesy, Neal and Band: "Chordate ," Philadelphia, The Blakiston Company.) mated so that the nervous impulse passes from one fibril to another in a manner analogous to that in which an electric current passes from one to another of two wires which are merely in contact, but not con- tinuous, or even "sparks" across a small gap. This type of relation between fibrils of two neurons is known as a synapse (Fig. 133; cells in spinal cord). There is accumulating evidence that transmission across a synapse is effected by instantaneous secreting of a specific chemical substance by the fibrils of the neuron which is sending the impulse across, this secreted "neurohumor" thus acting chemically on the adjacent fibrils of the other neuron. Central Nervous Organs Some invertebrates, such as arthropods and annelid worms, have a dorsal cephalic ganglion or "brain," but the central nervous organ in the body is a cord, or pair of cords, situated ventrally to the diges- tive tube. In flatworms there are one or more pairs of nerve-cords placed more or less laterally. In no invertebrate do the central nervous organs resemble closely those of vertebrates. Of all vertebrate organs, the central nervous organs, brain and spinal cords are most constant in manner of embryonic development, in position, and in general features of structure. Along the entire length of the middorsal line of the early embryo, a band of the outer layer (ectoderm, which otherwise gives rise to the outer layer, epidermis, of the skin) becomes infolded, detached from the outer ectoderm, and.
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