NERVOUS SYSTEM 



369 



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 



F.B 



M.B 



op.t\ I pt.sv m. rn.o IV 



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 Mnnro ; H.B, hind-brain ; in, 

 infundibulum ; Z.r, 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, para- 

 physis ; pc, prosocoele ; pn.o, pineal organ ; p.o, parietal organ ; pr, prosen- 

 eephalon ; pt, pituitary body ; rh, rhinocoele ; sp.c, spinal cord ; s.v, saccus vascu- 

 losus ; th, thalamencephalon ; iii, iv, third and fourth ventricles. (After Parker 

 and Haswell.) 



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 



VOL. VII 2 B 



