CHAPTER XV 

 MIDBRAIN 



IN THE preceding chapters the structure of the rhombencephalon 

 and the functions intrinsic to it were summarized. We now pass 

 to the cerebrum, with a radically different type of organization, 

 dominated by the optic and olfactory systems. In lower vertebrates 

 the major optic terminals in the midbrain evidently have determined 

 the course of its differentiation ; but, since optic fibers terminate also in 

 the diencephalon, a separate chapter is devoted to the visual system 

 as a whole. 



DEVELOPMENT 



Because the mesencephalon is the site of a remarkable series of 

 changes in the shape of the brain in early developmental stages, it is 

 appropriate to review here some features of this growth. Attention 

 has been called (p. 117) to the significance of von Kupffer's anterior 

 and posterior intraencephalic sulci as stable landmarks in the develop- 

 ment of all vertebrate brains. These mark the loci of two strong 

 cerebral flexures, which are extremely variable in different species 

 and among individuals of the same species. These flexures so modify 

 the topographic arrangement of parts as to make morphological 

 analysis difficult. The posterior sulcus of von Kupffer (the develop- 

 ment of which is described in chap, xiii) marks the posterior limit of 

 the great mesencephalic flexure, which bends the neural tube ventral- 

 ward and backward so that, at the close of the early swimming stage 

 of Amblystoma tigrinum (Harrison's stages 36-38), the hypothala- 

 mus is closely appressed against the ventral surface of the peduncle 

 and isthmus ('38, figs. 1-4, 15-18). The tuberculum posterius marks 

 the ventral fulcrum around which this bend is made. 



Beginning somewhat later than this ventral bending, there is a 

 flexure in the reverse direction more anteriorly at the site of von 

 Kupffer's anterior sulcus, which bends the cerebral hemispheres for- 

 ward and upward. In the early feeding stage (Harrison's stages 44, 

 45) this movement is far advanced ('386, figs. 1, 2); and from this 

 stage onward to the adult stage these flexures appear to be somewhat 

 straightened and masked by intussusception of newer tissues. 



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