. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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 it
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative; Vertebrates -- Anatomy. 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; JTr FIG. 111.—BiiAix OF Galeus canis, in situ. (Dorsal view.) (After Rohon.) Lol, olfactory lobe ; Tro, olfactory tract ; VII, prosericeplialon, showing at f/i a foramen for Wood-vessels ; ZH, thalamencephalon ; MH, meseneephalon ; HFT, metencephalon ; NH, myelencephalon ; R, spinal cord ; //, optic nerve ; ///, oculomotor ; IV, trochlear nerve ; V, trigemhml ; L, Trig, trigeminal lobe ; C,rcst, restiform body; IX, glossopharyngeal; A', vagus ; , eminentire 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 (Fi^r. 112, J~L\ are present on the floor of the thalamencephalon. They probably arise in connection with the inftmclibulum, 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 Rays it is more compressed and triangu- lar. In electric Rays a pair of lobi electric! arise from the gnu matter of the floo
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