1 1 2 The Nervous System 



velopment of the paired eyes, being best preserved in reptiles 

 as "an outcome of the life-habit which concealed the animal in 

 sand or mud, and allowed the forehead surface alone to protrude, 

 the median eye thus preserving its ancestral value in enabling the 

 animal to look directly upward and backward." This theory 

 receives no support from the structures seen in the fishes. 



In none of the fishes is the epiphysis more than a nervous 

 enlargement, and neither in fishes nor in amphibia is there 

 the slightest suggestion of its connection with vision. It seems 

 probable, as suggested by Hertwig and maintained by Dean 

 that the original function of the pineal body was a nervous one 

 and that its connection with or development into a median 

 eye in lizards was a modification of a secondary character. On 

 consideration of the evidence, Dr. Dean concludes that "the 

 pineal structures of the true fishes do not tend to confirm the 

 theory that the epiphysis of the ancestral vertebrates was con- 

 nected with a median unpaired eye. It would appear, on the 

 other hand, that both in their recent and fossil forms the epiphy- 

 sis was connected in its median opening with the innervation 

 of the sensory canals of the head. This view seems essentially 

 confirmed by ontogeny. The fact that three successive pairs of 

 epiphyseal outgrowths have been noted in the roof of the thala- 

 mencephalon* appears distinctly adverse to the theory of a 

 median eye." t 



The Brain of Primitive Fishes. The brain of the hagfish 

 differs widely from that of the higher fishes, and the homologies 

 of the different parts are still uncertain. The different ganglia 

 are all solid and are placed in pairs. It is thought that the 

 cerebellum is wanting in these fishes, or represented by a narrow 

 commissure (corpus restiforme) across the front of the medulla. 

 In the lamprey the brain is more like that of the ordinary fish. 



In the lancelet there is no trace of brain, the band-like spinal 

 cord tapering toward either end. 



The Spinal Cord. The spinal cord extends from the brain to 

 the tail, passing through the neural arches of the different ver- 

 tebrae when these are developed. In the higher fishes it is cylin- 



* The thalamencephalon or the interbrain is a name given to the region of 

 the optic thalami, between the bases of the optic lobes and cerebrum 

 f Fishes Recent and Fossil, p. 55. 



