258 AN INTRODUCTION TO ZOOLOGY 



Spinal Cord. 



The spinal cord calls for but brief notice. It is continuous 

 in front with the brain, and thence passes backwards in the neural 

 canal as a cylindrical column, slightly flattened dorso-ventrally 

 and gradually diminishes in size until it terminates at the end of the 

 caudal vertebrae. Above, it bears a shallow dorsal fissure, and 

 beneath a deeper ventral fissure. In transverse section it exhibits 

 the two characteristic varieties of nervous tissue, an inner mass of 

 grey matter, but not so definitely divided into dorsal and ventral 

 cornua as in Rana. Near the middle the canalis centralis will be 

 seen as an oval tube lined by a typical epithelium, which is ciliated. 

 Running through both grey and white matter are certain supporting 

 elements termed neuroglia cells, which are non-nervous. 



Cranial Nerves. 



We find in Scyllium the same cranial nerves that have 

 already been dealt with in Rana, and there is in addition a tiny 

 pair * arising from the front end of the prosencephalon, termed the 

 nervi terminales, which are also present, though very small, in the 

 frog. Indeed, the same ten nerves are to be found in all the 

 Craniata, arising from the same place and with the same general 

 distribution, although in higher forms, e.g. mammals, there are added 

 to these two more pairs, making twelve in all. The same is probably 

 true of the nervus terminalis, although it is not always so easy to 

 make out as in Scyllium, and it is, as yet, not very well known. 

 It is important, therefore, that these nerves and their point of origin 

 should be borne in mind, and for this reason they are set forth in 

 tabular form below. 



ORIGIN OF THE CRANIAL NERVES IN SCYLLIUM. 



* There is also a small nervus septalis arising behind the olfactorius, but 

 it is omitted here on account of its small size. 



