Pattern formed at upper cervical spinal cord levels by sensory fibers of spinal and cranial nerves; relation of this pattern to associated gray matter.

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RECENT studies of the spinal tract (or descending root) of the trigeminal nerve in human fetuses 1 have demonstrated that a considerable number of fibers from all three divisions of this nerve pass throughout the first cervical segment of the spinal cord at the age when the embryo first responds to exteroceptive stimulation in the circumoral region (7½ weeks of menstrual age *). Less than a week later (at just over 8 weeks of menstrual age; Humphrey, 5 Fig. 12) large numbers of fibers from all divisions of the spinal tract of the trigeminal, or fifth cranial, nerve (V) have been observed to terminate in the middle third of the second cervical segment. Although some fibers in the spinal tract of V have been traced as far as the upper levels of the fourth cervical segment at a later fetal age (8½ weeks; Humphrey 5 ), the great majority of the trigeminal