. The biology of the amphibia. Amphibians. 354 THE BIOLOGY OF THE AMPHIBIA represent a true nerve net inherited from invertebrate ancestors. In the frog, however, Cole (1925) has shown that there is a differentiation within this plexus of axons and dendrites, the neurons anastomosing only by their dendrites. Further, only a few nerve cells enter into these fusions, which seem to be a secondary modification rather than a primitive inheritance. Reflex Arc.—Neurons are arranged in functional units, the reflex arcs, each with a neuron receiving the impulse from the. Fun . Lat. Fig. 124.—Diagrammat
. The biology of the amphibia. Amphibians. 354 THE BIOLOGY OF THE AMPHIBIA represent a true nerve net inherited from invertebrate ancestors. In the frog, however, Cole (1925) has shown that there is a differentiation within this plexus of axons and dendrites, the neurons anastomosing only by their dendrites. Further, only a few nerve cells enter into these fusions, which seem to be a secondary modification rather than a primitive inheritance. Reflex Arc.—Neurons are arranged in functional units, the reflex arcs, each with a neuron receiving the impulse from the. Fun . Lat. Fig. 124.—Diagrammatic cross-section of the spinal cord of a larval salamander showing the relation of the sensory to the motor neurons. , correlation neuron; , dorsal root; , fasciculus longitudinalis medialis; , funiculus lateralis; , gray matter; My., myotome; S., skin; , spinal ganglion; , tractus bulbo-spinalis; , tractus tecto- spinal; , tractus spino-bulbaris; , tractus spino-cerebellaris; , tractus spino-tectalis; , ventral horn neuron; , ventral root. {After Herrick and Coghill.) sense organ, another neuron transmitting it to an effector organ, and, generally, a third intercalated between the two (Fig. 124). Reflex arcs are usually complicated by the addition of several cells of this third category; twigs from their axons, called "col- laterals," making possible the transmission of impulses to several adjustors or effectors. The passage of an impulse through a reflex arc requires more time than is consumed by the impulse traveling the same length of nerve and stimulating the same end organ alone. Hence, the synapse and the myoneural junction apparently present some functional modification in the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may n
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