SKATE. 277 



laterally, corresponding with the elongated nasal pit. In some Rays 

 they are secondarily lobed. The olfactory tracts vary much in length in 

 the Elasmobranchii. They are said to be wanting in Raja miraletus. 



b. The cerebral lobes or main part of the fore-brain. They are much 



compressed dorso-ventrally, triangular in shape, with the outer angles 

 swollen where the olfactory tracts take origin. They are slightly 

 notched in front and grooved ventrally, indicating their bilaterally 

 symmetrical structure. Lateral ventricles are wanting, but the degree 

 to which they are obliterated among Elasmobranchii is very variable. 

 A triangular spot, shaded in the diagram, behind the cerebral lobes 

 marks the position of the third ventricle. The filamentous pineal 

 gland (Epiphysis cerebri) has been removed. 



c. The mid-brain, corpora bigemina, or optic lobes. These bodies are 



large and hollow, and lie above the aquaeductus Sylvii or passage 

 between the third and fourth ventricles, with which their cavities 

 communicate. 



d. The cerebellum, or roof of the anterior portion of the fourth ventricle, 



the ventricle of the hind-brain. It is large in size in all Elasmobranch 

 fishes, and often complexly convoluted. It consists here of two 

 lobes in front and a long triangular lobe behind. It contains a 

 large cavity, freely open to the fourth ventricle, which is just visible 

 behind it, being for the most part covered by the posterior triangular 

 lobe. 



e. The convoluted corpora restiformia with which are connected the roots 



of the fifth, facial, and auditory nerves. 



f. The commencement of the spinal cord. The medulla oblongata or the 



sides and floor of the hind-brain is remarkably short in the Rays. It 

 is usually elongated in Elasmobranchii. 



g. Optic nerves. There is a superficial chiasma to these nerves in Elas- 



mobranchii, Ganoidei) and Dipnoi. 



h. Third nerve or oculomotor. It springs from the base of the mid-brain, 

 but its superficial origin is hidden by the saccus vasculosus. 



i. Fourth nerve or trochlearis. It arises, as in all Vertebrata, from the 

 roof of the aquaeductus Sylvii behind the mid-brain, and in front 

 of the cerebellum. 



j. Roots of the fifth (trigeminus), seventh (facial or ' portio dura '), and 

 eighth (auditory or 'portio mollis') nerves. The facial rises dorsally, 

 the auditory posteriorly and ventrally, while the remaining roots 

 belong to the trigeminus. This close connection of the nerves at 

 their roots is generally found in Pisces. The trigeminus and facial 

 are also closely connected in the Gasserian ganglion in Annra (Am- 

 phibia). The facial and auditory nerves also are often closely con- 

 nected in Amphibia and Reptilia at their roots'. 



