94 



THE NERVOUS SYSTEM 



We may mention as an example of a reflex arc involving many segments of 

 the 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 cord 

 in the low cervical region, the skin covering the dorsal aspect of the thorax be 

 stimulated by pulling lightly on a hair, the hind limb of the corresponding side 

 begins 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 the 

 corresponding neural segment; (2) a long descending association neuron from the 



Fig. 69. Diagram of the spinal arcs involved in the scratch-reflex: Ra and Rp, Receptive 

 paths from hairs in the dorsal skin of left side; Pa and P/3, association neurons; FC, motor fibers of 

 ventral root. (Sherrington.) 



shoulder to the leg segments, and (3) a primary motor neuron to a flexor muscle 

 of the leg (Fig. 69). 



A primary motor neuron seldom, if ever, belongs exclusively to one arc, but 

 serves as the final channel to which many streams converge. Its perikaryon 

 gives off wide-spread dendritic processes, through which it comes into relation 

 with the ramifications of axons from many different sources. In this way 

 impulses reach it from the dorsal roots, and from the fasciculi proprii of the 

 spinal cord, as well as from a number of tracts which descend into the spinal 

 cord from centers in the brain (the corticospinal, rubrospinal, tectospinal, and 

 vestibulospinal tracts). The primary motor neuron is, as Sherrington has said, 

 "the final common path." 



