THE SEGMENTAL VALUE OF THE CEANIAL NERVES. 133 



1. Segmental nerves develop at a very early stage as outgrowths 

 from the neural ridge on the dorsal surface of the brain. 



2. At an early period they shift downwards, and acquire new or 

 secondary roots of attachment to the sides of the brain. 



3. The general course of the main stem of a segmental nerve is at 

 right angles, or nearly so, to the axis of the head at the point of 

 origin of the nerve. This feature, which is explained more fully in 

 the paper quoted above, is evident from an inspection of fig. 17, in 

 which the directions of the segmental nerves are shown, and from the 

 consideration that the course of segmental nerves must be approxi- 

 mately parallel to the boundaries of the segments to which they 

 belong : a segmental nerve could not run transversely across a number 

 of segments. 



4. Segmental nerves have the characteristic relations to the visceral 

 clefts and arches, and, therefore, also to the head cavities in these 

 arches, first pointed out by Stannius as noticed above, each nerve 

 supplying the borders of one cleft, and therefore of two arches. 

 Concerning this test, it may be noted that, although from the 

 constancy of the relations of visceral clefts to other structures in all 

 vertebrates above Amphioxus, there can be no doubt that Gegenbaur, 

 Huxley, Semper, and others are correct in maintaining the segmental 

 value of these clefts, yet that the total absence of any correspondence 

 between the visceral clefts and the body segments in Amphioxus, and 

 still more in the Ascidians, makes it very doubtful whether this 

 segmental character is a primitive one. 



5. Segmental nerves very constantly present ganglionic enlarge- 

 ments, either at or near their points of division into their two main 

 ventral branches. 



Having thus cleared the ground, and explained what we mean by a 

 segmental nerve, and why it is of importance to determine which of 

 the cranial nerves are of segmental value, and which are not, I propose 

 to consider these nerves and discuss their claims in order, beginning 

 with the most anterior ones, and taking them in the sequence usually 

 adopted by anatomists. 



I. The First or Olfactory Nerve, — This nerve was until recently 

 supposed, by reason of its development, to stand quite apart from the 

 rest of the cranial nerves, and to be, properly speaking, a part of the 



