152 NERVOUS SYSTEM OF VERTEBRATES. 



brain tube it became necessary for the retinal area to project 

 toward the skin in order to receive light stimuli. This area con- 

 tained both primary receptive cells and brain centers. As it was 

 carried out from the brain and the optic vesicle was formed, the 

 stalk of the vesicle consisted chiefly of the fibers of the optic tract. 

 Since these fibers were destined to decussate in the ventral wall 

 of the brain, it was advantageous for the stalk to shift ventrad 

 and so allow the tract to become shorter. This is illustrated in 

 an accompanying diagram (Fig. 75). Although this special visual 

 organ has been developed, the general cutaneous endings retain 

 their sensitiveness to Hght in varying degrees. 



In Amphioxus special light percipient cells are contained in 

 the front end of the brain and throughout the spinal cord. These 

 have been shown to be similar to the light percipient cells in the 



Epiphysis 2 



Tectum 



y Comm. 

 Paraphysis \V\ , Post. 



\ ^Nuc. haben. 

 Velum- 



FlG. 76. — A sketch showing the relations of the two epiphyses in vertebrates. 



gangha of certain worms. Whether there is any relation be- 

 tween the hght percipient cells of Amphioxus and the lateral eyes 

 of vertebrates is uncertain. 



In addition to the lateral eyes two other organs of light percep- 

 tion are present in vertebrates, the degenerate or reduced pineal 

 eyes. Although in most vertebrates only one of these structures 

 is present, at least in the adult, both are present in adult cyclos- 

 tomes, and in some reptiles one in front of the other. The 

 position and innervation of the organ found in other classes 

 of vertebrates show that sometimes it is the anterior and 

 sometimes the posterior organ which is present. The anterior 

 organ sends its nerve fibers into the nucleus habenulae or adjacent 

 center. Those from the posterior organ go to the region of the 

 posterior commissure and probably enter the tectum opticum. 



