597 



end in the vagus lobe are concerned in the innervation of the mucosa 

 of the mouth and branchial apparatus, and also send rami to the in- 

 testine, heart, and other viscera. No sensory fibres of the spinal 

 nerves supply visceral structures. We know of no sensory fibres 

 entering the spinal cord from the sympathetic system. It is possible 

 that we may find a homologue in rudimentary or transient structures^ 

 such as the transient apparatus of Beard ('96) in Selachians and 

 Ganoids, or the giant cells described by various authors in Amphioxus,. 

 Petromyzon, and several fishes. 



The facts here presented are of interest in connection with the 

 question of the segmentation of the brain. Obviously, if the nerves 

 of the V-VIII-lateral line group are alone homologous with the dorsal 

 roots of the spinal nerves, these alone can be compared with those 

 roots in determining segmentation. It seems that we have the sensory 

 innervation of the viscera restricted to cranial centers. This may have 

 come about by a process of receding from the trunk region, the 

 homologue of the visceral centers being now wanting or rudimentary 

 in that region. In the integument, also, we find certain organs along 

 the whole trunk, the canal organs, innervated from a cranial center. 

 This is, however, a very different case. The cranial center for the 

 lateral line is similar in structure to the trunk centers for sensory 

 nerves, and some of the lateral line fibres reach the most anterior of 

 the trunk centers. Further, we have some evidence (Beaed '85) that 

 the lateral line organs reach their trunk position by the backward 

 growth, during development, of a fundament of cephalic origin. In 

 using the cranial nerves to determine segmentation, three courses 

 remain open: first, to disregard the visceral nerves, and count as 

 many segments as there are integumentary nerves, which would con- 

 flict with the results from the study of head cavities and branchio- 

 meres; second, to count both sets of nerves, which leaves us without 

 any means of comparing the head with the trunk, in which the vis- 

 ceral nerves are wanting; or third, to disregard all sensory nerves^ 

 and consider only the motor nerves. The last course has three ad- 

 vantages: the motor nerves are comparatively constant in the verte- 

 brate series, they hold constant relations with the head cavities and 

 branchiomeres, and they may be directly compared with the spinal 

 nerves. The direct comparability of the motor nerves in the cranial 

 and trunk regions is what we should expect from our knowledge of 

 the muscular system. That there should be great differences in the 

 sensory nerves in various regions of the body, as well as between 

 different vertebrates, is likewise to be expected from the great 



