150 NERVOUS SYSTEM OF VERTEBRATES. 



situated in the lateral wall of the thalamus and is known as the 

 corpus geniculatum later ale. The morphology of the thalamic 

 and midbrain optic centers and their tertiary connections in higher 

 vertebrates will be discussed in a later chapter (Chap. XVI). 



The question of the origin of the eye and of its relation to other 

 sensory systems may now be reviewed. Obviously a close rela- 

 tion would not be expected between the eye and the nerves or 

 organs of visceral sensation. Is the eye, then, related to cutaneous 

 sense organs and has it been formed by modification of any pre- 

 existing organ, or is it a wholly new structure ? The facts show a 

 remarkable similarity between the eye and the general cutaneous 

 structures of other segments. The retina is derived from the 

 dorso-lateral wall of the second neuromere. It therefore repre- 

 sents a dorso-lateral nucleus and as such corresponds to the 

 cutaneous nuclei in the hindbrain and spinal cord. The optic 

 tract which arises in the retina is a secondary brain tract for which 

 the tectum opticum is the secondary nucleus. The secondary 

 tract decussates in the ventral wall of the brain as do the secondary 

 tracts from the cutaneous nuclei, and ends in the same center 

 with those tracts, the tectum opticum. The tectum itself is a 

 primary cutaneous center, since in several classes of vertebrates 

 it receives a part of the sensory trigeminus nerve, and in the 

 embryos of lower vertebrates the ophthalmicus profundus nerve 

 arises from the roof of the midbrain. In lower vertebrates the 

 mode of formation of the optic vesicle bears a significant resem- 

 blance to the mode of formation of the general cutaneous ganglia. 

 In selachians, for example, both are formed as hollow outgrowths 

 from the dorso-lateral wall of the neural tube. 



If to these facts there be added now the physiological facts 

 with which this chapter was begun, a remarkably strong body of 

 evidence is presented for a relationship between the eye and the 

 general cutaneous sensory system. The general cutaneous nerves 

 are susceptible to light stimulation and the impulses carried to the 

 brain produce reactions which correspond to those produced when 

 the eyes are the vehicles of light perception. Whether the frog 

 has the use of its eyes or only of its skin it turns its head toward 

 the light and jumps toward it. Not only are the cutaneous nerves 



