Herrick, Subdivision of the Brain. 401 



The primary longitudinal functional zones have likewise suffered 

 extreme distortion by reason partly of the development of the 

 organs of special sense but more especially on account of the 

 elaboration of the massive organs of correlation. 



The diencephalon and the mesencephalon, from our present 

 point of view, are not natural regions, nor can any other transverse 

 division of the brain be made which will satisfy the conditions, for 

 the primary metamerism has ceased to be an important factor 

 in the problem. The terms mid-brain, thalamus, etc., will in 

 any event of course continue in use as convenient topographic 

 designations. But from the point of view of a broad compar- 

 ative morphology, I believe that they are confusing rather than 

 helpful. 



Functionally and genetically, the retinae, optic nerves, chiasma 

 and tracts and the optic thalamus (sensii stricto) should be asso- 

 ciated with the optic tectum of the mid-brain to form an ophthal- 

 mencephalon whose boundaries cross freely those of the classic 

 encephalic regions. 



The principle upon which this term is based is the same as that 

 which led His to adopt (in the BNA) the term fasciculus cerebro- 

 spinalis in place of the older terms, pyramid and pyramidal tract; 

 viz: the association of neurones belonging to the same functional 

 system. The advantages of this usage over any purely topograph- 

 ical designation are so clearly brought out in the discussion of the 

 terms pyramids, etc., that we refer the reader here to the words of 

 His. 3 



The term ophthalmencephalon as proposed here is analogous 

 with rhinencephalon as used by the BNA for the whole olfactory 

 apparatus of the forebrain except the olfactory portion of the 

 pallium.^ 



Aside from the visual centers, there are in the midbrain the 

 pedunculus cerebri, coUiculus inferior and other correlation cen- 

 ters less clearly defined and still imperfectly known. It would be 

 premature in the present state of our knowledge to attempt a final 

 detailed subdivision of this difficult part of the brain; but the follow- 

 ing simple outline may serve as a working basis. 



In the mammalia the part of the brain between the tectum opti- 

 cum and the cerebellum may best be divided into two regions, the 



3 Archivf. Anat. [u. Physiol], Suppl. Bd., 1895, p. 163. 



* On the rhinencephalon, BNA, see beyond under the subdivision of the forebrain. 



