CHAPTER XIV 



NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE 



THE nervous system consists of the brain and the spinal cord, 

 and of the cranial and spinal nerves. The rudiment of the 

 future brain and spinal cord first appears in the embryos of 

 some Cyclostomes (e.g. JBdellostoma), of Elasmobranchs, and of 

 Chondrostei (e.g. Acipenser}, and of Neoceratodus among the Dipnoi, 

 in the form of a tubular medullary canal pinched off from the 

 epiblast of the dorsal surface of the body. By a somewhat 

 different method, but with the same final result, a medullary 

 canal is formed in other Cyclostomes (e.g. Petromyzon), in the- 

 Holostei and Teleostei, and in Lepidosiren, 1 from a solid in- 

 growing keel of epiblast which subsequently becomes tubular. 

 Later, the medullary canal in the head enlarges, and becomes 

 divided by two transverse constrictions into three vesicles, the 

 primary fore-, mid-, and hind-brain, leaving the rest of the canal 

 to form the spinal cord. 



The Spinal Cord. This portion of the medullary canal 

 retains a simpler and more uniform cylindrical structure. Its 

 walls thicken and their component cells become converted into 

 nerve cells and nerve fibres, but a remnant of the original 

 cavity remains in the adult as a minute axial canal, with a 

 ciliated epithelial lining, the central canal of the spinal cord or 

 tnyelocoele. In most Fishes the spinal cord extends the whole 

 length of the body, but in some Teleosts, especially in certain 

 Plectognathi, it is remarkably short. In a Sun-Fish (Orthago- 

 riscus), 2^ metres long, and weighing about a ton and a half, the 

 cord was only 15 mm. in length, or shorter than the brain. 



The Brain. At an early stage in its embryonic history the 



1 Graham Kerr, Quart. Journ. Micr. Sci. xlvi. 1902, p. 417. 

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