{369}[Illustration: FIG. 210.—Diagram of the general structure of the brain in Craniates. A, vertical longitudinal section; B, dorsal view showing the brain cavities on the right side. _c_, Cerebellum; _c.c_, central canal of the spinal cord; _c.h_, cerebral hemispheres; _c.s_, corpus striatum; _F.B_, fore-brain; _f.m_, foramen of Munro; _H.B_, hind-brain; _in_, infundibulum; _l.v_, lateral ventricle; _m_, mesocoele; _M.B_, mid-brain; _m.o_, medulla oblongata; _o.l_, olfactory lobe; _op.l_, optic lobe; _op.t_, optic thalamus; _p_, paraphysis; _pc_, prosocoele; _pn.o_, pineal organ; _p.o_, parietal organ; _pr_, prosencephalon; _pt_, pituitary body; _rh_, rhinocoele; _sp.c_, spinal cord; _s.v_, saccus vasculosus; _th_, thalamencephalon; iii, iv, third and fourth ventricles. (After Parker and Haswell.)] Scarcely less complicated, and perhaps even more interesting from a morphological standpoint, are the structures arising out of the thalamencephalon. By thickenings of its lateral walls two large ganglia, the _optic thalami_, are formed, and on the inner or dorsal aspect of each of these a _ganglion habenulae_ is developed. From the sides of the thalamencephalon the primary _optic vesicles_ are derived, which later become transformed into the retinal parts of the paired eyes and the optic nerves. Besides the optic vesicles there is a second pair of embryonic outgrowths which arise from the roof of the thalamencephalon. These outgrowths form stalked vesicles and represent a pair of degenerate visual {370}organs. Usually they become so displaced that the left one lies in front of the right, and they appear as if median. The subsequent fate of the vesicles differs greatly in different Craniates. Both persist in the Lamprey, the right vesicle to some extent retaining its primitive visual function as a _parietal eye_ and directly overlying the left or pineal vesicle. In Elasmobranchs the two unite to form a glandular organ, the so-called _pineal body_ of the adult, and in Teleosts the left vesicle disappears, leaving the right as a pineal body.[441] There is also an embryonic median outgrowth from the roof of the prosencephalon, the _paraphysis_, which soon disappears and whose significance is not known. A median hollow downgrowth from the floor of the thalamencephalon forms the _infundibulum_, which becomes attached to a caecal diverticulum from the roof of the mouth. With rare exceptions the diverticulum loses all connexion with the mouth, and, as the _pituitary body_ or _hypophysis_, it appears as an appendage to the extremity of the infundibulum. In the Crossopterygii the connexion is retained even in the adult by means of a slender canal extending from the pituitary body and opening into the oral cavity. Laterally, the base of the infundibulum grows out into a pair of rounded lobes, the _lobi inferiores_, and distally into a thin-walled glandular sac, the _saccus vasculosus_, which lies just behind the pituitary body.