. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . -placeat the mid-ventral line is calledthe ventral commissure. § 6. The axons thus far de-scribed may now be groupedinto three classes: (1) the ven-tral root axons, which arisefrom cells in the ventral part ofthe tube and pass out, as mo-tor fibres, connecting the tubewith muscles; (2) the dorsalroot axons, which arise fromcells in the spinal ganglia, andby their two axons, connect thetube with sense-organs; and (3)the central axons, which liewholly w
. Elements of physiological psychology; a treatise of the activities and nature of the mind, from the physical and experimental points of view . -placeat the mid-ventral line is calledthe ventral commissure. § 6. The axons thus far de-scribed may now be groupedinto three classes: (1) the ven-tral root axons, which arisefrom cells in the ventral part ofthe tube and pass out, as mo-tor fibres, connecting the tubewith muscles; (2) the dorsalroot axons, which arise fromcells in the spinal ganglia, andby their two axons, connect thetube with sense-organs; and (3)the central axons, which liewholly within the tube, andconnect one part of it withanother (compare Fig. 15).There is still a fourth class ofaxons to be added. Some of thecells in the neural crest, insteadof remaining near the dorsal partof the tube and sending axonsinto it, migrate in a ventral di-rection and develop into nerve-cells; but these send out only a singleaxon, which passes to some of the organs of the interior of the body—the stomach, intestines, pancreas, salivary glands, heart, and blood-vessels—and enters into connection with muscular or glandular tissue. Fig. 15.—Development of the Nerve-Cells ofthe Spinal Ganglia. (Cajal.) The drawingis from an embryo chick. A is the spinalcord, containing d, a motor nerve-cell, fromwhich issues an axon; this with other simi-lar axons emerges from the cord into B, theventral root (the connection between thisroot and the cord has been accidentally,broken in this section). C is the dorsal root,consisting of axons which issue from thespinal ganglion Z>. Axons issuing from thisganglion in the opposite direction join thoseof the ventral root, to form E, a spinal the spinal ganglion are seen cellswhich are still bipolar, h; others, i, which arebecoming transformed into unipolar cells;and others, as % which are already distinctlyunipolar. ^ is a sympathetic ganglion,showing cells with axons, a, which join thespinal nerve, and others with axons, e,
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