The anatomy of the nervous system, from the standpoint of development and function . mechanism the sensory fibers arise from giant cells locatedwithin the spinal cord and that the ventral root fibers are collaterals from the central motortract. In adult Amblystoma these sensory and motor elements are replaced by the usualtype of primary sensory and motor neurons. 94 THE NERVOUS SYSTEM We may mention as an example of a reflex arc involving many segments ofthe cord the scratch-reflex of the dog, which has been very carefully investi-gated by Sherrington (1906). If, some time after transection of
The anatomy of the nervous system, from the standpoint of development and function . mechanism the sensory fibers arise from giant cells locatedwithin the spinal cord and that the ventral root fibers are collaterals from the central motortract. In adult Amblystoma these sensory and motor elements are replaced by the usualtype of primary sensory and motor neurons. 94 THE NERVOUS SYSTEM We may mention as an example of a reflex arc involving many segments ofthe cord the scratch-reflex of the dog, which has been very carefully investi-gated by Sherrington (1906). If, some time after transection of the spinal cordin the low cervical region, the skin covering the dorsal aspect of the thorax bestimulated by pulling lightly on a hair, the hind limb of the corresponding sidebegins a series of rhythmic scratching movements. By degeneration experi-ments it was shown that this reflex arc probably includes the following elements:(1) a primary sensory neuron from the skin to the spinal gray matter of thecorresponding neural segment; (2) a long descending association neuron from the. Fig. 69.—Diagram of the spinal arcs involved in the scratch-reflex: Ra and R3, Receptivepaths from hairs in the dorsal skin of left side; Pa and Pft, association neurons; FC, motor fibers ofventral root. (Sherrington.) shoulder to the leg segments, and (3) a primary motor neuron to a flexor muscleof the leg (Fig. 69). A primary motor neuron seldom, if ever, belongs exclusively to one arc, butserves as the final channel to which many streams converge. Its perikaryongives off wide-spread dendritic processes, through which it comes into relationwith the ramifications of axons from many different sources. In this wayimpulses reach it from the dorsal roots, and from the fasciculi proprii of thespinal cord, as well as from a number of tracts which descend into the spinalcord from centers in the brain (the corticospinal, rubrospinal, tectospinal, andvestibulospinal tracts). The primary motor neuron is, as
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