I20 NERVOUS SYSTEM OF VERTEBRATES. 



In higher vertebrates the acusticum and the cerebellum become 

 more and more higher specialized in their histological structure 

 and secondary connections. Since this speciahzation is largely 

 due to the fact that these are the centers for the special cutaneous 

 (acustico-lateral) system of nerves, the special structure and the 

 course of differentiation will be described in the next chapter. 

 At the same time that the specialization is going on the general 

 cutaneous fibers for the most part cease to enter the acusticum 

 and cerebellum and come to be concentrated in the spinal V tract, 

 so that in higher vertebrates by far the greatest part of the cutaneous 

 fibers end in the nuclei of the spinal V tract and of the dorsal 

 funiculi. While this concentration of the primary tracts and 

 nuclei is going on, a change takes place in the secondary connections 

 as well. The secondary tract from the spinal cord and the nucleus 

 funiculi, the lemniscus, now ends only in part in the roof of the 

 mesencephalon. A larger part of its fibers end in nuclei situated 

 in the lateral walls of the diencephalon. (See Chapter XVI.) 



We should expect the general cutaneous system of nerv'es 

 to be the most primitive and widespread part of the nervous 

 system, unless the general visceral nerves be excepted. And so 

 it is, but there is reason to think that even in the lowest vertebrates 

 the system has undergone considerable modification. A pair 

 of cutaneous nerv^es and corresponding centers is to be expected 

 in each segment, but in cyclostomes the first five segments of the 

 brain are without cutaneous roots and the corresponding area 

 of the skin is supplied by nerves whose roots enter the brain in 

 the sixth neuromere. The fact that one of these, the ophthal- 

 micus profundus, arises in the embryo from the fifth neuromere 

 is taken as evidence that it once held that position in the adult. 

 Even so, the first four segments of the head are without cutaneous 

 nerves of their own and are supplied by cutaneous rami coming 

 from more caudal segments. In selachians, however, the N. 

 terminahs connected vdth the first neuromere is probably the 

 cutaneous nerv^e of that segment. Reasons will be given in a 

 later chapter (Chapter VIII) for thinking that the cutaneous nerves 

 of the second, third and fourth segments have been modified 

 into visual organs, while the ophthalmicus profundus has come 



