134 



SUB-CLASS ELASMOBRANCHII. 



differ considerably as to the length of the thalamencephalon.* 

 In some, this part of the brain is much elongated, being com- 

 pletely exposed in its dorsal part and passing without break 

 into the anteriorly-placed cerebrum {Notidanidae, Spinacidae, 

 Scymnidae and most Plagiostomes). In others the thala- 

 mencephalon is very short and its dorsal surface is almost com- 

 pletely covered by the cerebrum, the hinder part of which 

 almost, if not quite, touches the optic lobes {Carcharias, Sphyrna, 



Oxyrrhina, Galeus, Trygon). It would 

 appear that the latter condition is develop- 

 mentally the most primitive, for brains with 

 a long thalamencephalon in the adult, e.g. 

 Scyllium, have in the later embryonic 

 stages a short thalamencephalon with the 

 cerebrum and optic lobes in contact over it. 



The cerebellum in some forms is very 

 large and may reach forward to the cere- 

 brum. The restiform tracts, or sides of 

 the medulla are much developed, and folded 

 or even convoluted. This is ascribed by 

 Burckhardt to the enormous development 

 of the sensorial centres in the dorso-lateral 

 parts of the spinal cord and brain in conse- 

 quence of the insertion of the nerves of 

 the lateral line organs. 



The dorso-median tract of non-nervoiis tissue 

 which is so often found along the central nervous 

 system of the Vertebrata and was first mentioned 

 by the physiologist Haller {Opera Minora, 1768, 

 bd. 3) has been detected as a septum in the cere- 

 bellum of many Plagiostomes. It is of course also present, as in all Ver- 

 tebrates, over the third and fourth ventricles and on the posterior wall 

 of the lateral ventricle. Very possibly the very thin dorsal wall — at 

 the bottom of the dorsal fissure — of the central canal of the spinal cord is 

 to be regarded as part of it, though there is here a little nervous 

 tissue. 



Cranial nerves t (see p. 72). There is nothing special to 



* This and other features of the brain seem to be influenced by the size 

 and position of the eyes, vide Burckhardt, British Association Reports, 1900. 



t H. Stannius, Das peripherifiche Nervensystem der Fische, Rostock, 1849. 

 J. C. Ewart, " On the Cranial Nerves of Elasmobranch Fishes," Proc. Roy. 

 Soc, 45, 1889, p. 524. Id., Supplementary note on the cranial nerves of 

 Elasmohranchs, Edinburgh, 1892. A. M. Marshall and W. B. Spencer, " On 



Fig. 78.— Brain of Car- 

 charias from above 

 (after Miklucho-Mac- 

 lay). SI olfactory lobe ; 

 Vh cerebrum ; Mh 

 optic lobe ; Uh cere- 

 bellum ; N restiform 

 tract ; Vag vagus. 



