One of the interesting features of the trigeminal nerve is its long caudal extension in the radix spinalis. The axons of the cells in the gasserian ganglion enter the pons and divide, one branch going to the main sensory nucleus, the other turning caudad and extending even as far as the first or second cervical segment of the spinal cord. The behavior of these fibers is far different from that of the homologous fibers in Lissauer's tract in the spinal cord. In the spinal cord these fibers run for a distance of one or at most two segments before entering the gray matter of the posterior horn whereas in the case of the spinal trigeminal tract they run for a much greater distance before terminating in the substantia gelatinosa of the bulb. The explanation for this is to be sought in the phylogenetic development of the trigeminal nerve.
FREEMAN W. THE RADIX SPINALIS TRIGEMINI AND THE PRINCIPLE OF USURPATION. Arch NeurPsych. 1926;15(5):607-612. doi:10.1001/archneurpsyc.1926.02200230072006