138 COMPARATIVE ANATOMY. 



and approximated together. In almost all Sharks the prosen- 

 cephalon is relatively much longer than any of the other regions. 

 The olfactory tracts, the length of which varies much, are con- 

 nected with the anterior end of the prosencephalon, and pass 

 forwards into the large olfactory lobes, from which the olfactory 

 nerves arise (Fig. Ill, Tro, Lol}. 



The thalamencephalon, appearing like a small commissure 

 wedged in between the prosencephalon and mid-brain, grows out 

 on its dorsal surface to form a chimney- or tube-like epiphysis ; 



Zol 



FIG. 111. BRAIN OF Galeus canis, in situ. (Dorsal view.) (After Rohon.) 



Lol, olfactory lobe ; Tro, olfactory tract ; VH, prosencephalon, showing at fn a 

 foramen for blood-vessels ; ZH, thalamencephalon ; MH, mesencephalon ; 7/77, 

 metencephalon ; Nil, myelencephalon ; 72, spinal cord ; 77, optic nerve ; 777, 

 oculomotor ; IV, trochlear nerve ; V, trigeminal ; L, Trig, trigeminal lobe ; 

 C,rcst, restiform body ; IX, glossopharyngeal ; A', vagus ; E,t, eminentise 

 teretes, between which is the calamus scriptorius. 



this may reach to such a length as to extend beyond the anter- 

 ior end of the brain for a considerable distance, and pass distally 

 into the roof of the skull. 



Two pairs of small folds, spoken of as lobi inferiores and sacci 

 vasculosi or processus infundibuli (Fig. 112, UL), are present on the 

 floor of the thalamencephalon. They probably arise in connection with 

 the infundibulum, or perhaps with the hypophysis also. 



The cerebellum is always very large, overlapping the medulla 

 oblongata to a greater or less extent : it is divided up into seve- 

 ral folds lying one behind the other (Fig. Ill, HIT). In Sharks 

 the medulla oblongata is an elongated cylindrical body (Fig. 

 Ill, NH}, while in Bays it is more compressed and triangu- 

 lar. In electric Rays a pair of lobi electrici arise from the gray 

 matter of the floor of the fourth ventricle, and these enclose a 

 mass of giant nerve-cells. For further details concerning, e.g., the 

 restiform bodies and trigeminal lobes, compare Figs. Ill and 1 1 '2. 



