104 Journal of Comparative Neurology. 



striatum is crossed and probably serves to make some special 

 connections. The dorsal path is distinguished by repeated de- 

 cussations in its course. The result of the complicated arrange- 

 ment is to ensure stimuli from either olfactory lobe reaching the 

 motor nuclei of both sides, which is not surely accomplished 

 by either the first or second paths. Even in the most primitive 

 craniates no part of this apparatus shows any essential likeness 

 to the structure of either the somatic or splanchnic sensory 

 division of the nervous system. 



This examination also shows that the olfactory has no claim 

 to be considered a segmental nerve. It is rather to be consid- 

 ered as a special nerve and its sense organ as the only really 

 special sense organ of peripheral origin (the eye being of cen- 

 tral origin and the ear belonging to the category of lateral line 

 organs). The olfactory organ and central apparatus lie, indeed, 

 in the first three head segments, but they do not serve in any 

 way to define those segments, since they have no points of 

 comparison with structures of typical segments. Toward the 

 definition of the more anterior head segments the discovery of 

 certain rudimentary structures has recently furnished promising 

 material. Of these, it is possible that the "accessory olfactory 

 nerves" described by Pinkus (i6) and Locy (14) belong to two 

 of the segments which enter into the primary fore brain, while 

 the N. thalamicus of Platt (17) belongs to the mid brain. The 

 latter nerve is probably a remnant of a general cutaneous nerve 

 which formerly had its central ending in what is now the tectum 

 opticum. The centers of the "accessory olfactory nerves," 

 however, have become so highly modified in the service of the 

 olfactory organ that they can no longer be intelligently com- 

 pared with the sensory centers of more caudal segments. It is 

 probable that they originally formed the most anterior portion 

 of the somatic sensory column. 



It is noteworthy that of the commissures in the mid, 

 'tween, and fore brain, the ventral ones (commissura ansulata 

 and decussatio postoptica) are directly comparable with the 

 ventral commissures of the spinal cord and medulla. Of the 

 dorsal commissures, on the other hand, only the dorsal decus- 



