DORSAL ASPECT OF THE MESENCEPHALON. 



441 



.y epen., mesen., meten. are abbreviations of diencephalon, etc. So much of the 

 preparation as is not included therein belongs to the prosencephalon. 



Pgn. Postgeniculatum. Tli. Thalamus. 



The sides of all the cceliae are beveled oft so as to expose their roofs more clearly. The 

 widest portion is the metacoelia, whose proper roof (metatela) is so thin that the laminae 

 of the overhanging cerebellum show through it . 



The epicoelia presents two very different portions a caudal, which is short but wide 

 and high, reaching up into the cerebellum (PI. II, Fig. 4), and a cephalic, longer but nar- 

 rower and lower, excepting at its cephalic end, where its roof, the valvula, rises to join 

 the postoptici. 



The succeeding contracted portion represents the mesoccelia ; the next mesal cavity 

 is the diaccelia, the roof of which presents the two parallel diaplexuses, diverging in the 

 aula to connect through the portae with the proplexuses. 



On the right (left of the figure) the crescentic line representing the transection of the 

 medico rnu should reach the end of the line indicating the boundary between the hemi 

 sphere and the postgeniculatum; see Fig. 121. 



The membranes and the relative areas of alba and cinerea are not shown. 



Fig. 114. Dorsal aspect of the mesencephalon, 

 exposed by the separation of the cerebellum and 

 hemispheres ; from Prep. No. 390, M. C. U. 



Objects. To expose (A) the dorsal aspect of the 

 mesencephalon, the cephalic aspect of the cerebel- 

 lum and the caudal aspect of the hemispheres ; (B) 

 the origin of the NN. trochleares from the cephalic 

 part of the valvula (vv.) ; (C) the caudal position and 

 retroversion of the conarium. 



Preparation. The fresh brain was carefully held 

 and the caudal portions of the hemispheres gently 

 pushed cephalad, the attachments to the mesencepha- 

 lon being divided with the tracer, until the conarium 

 was exposed ; then the cerebellum was in like man- 

 ner tilted caudad, care being had not to tear the 

 delicate valvula. 



By the above operations the natural encephalic 

 flexure was so exaggerated as to bring the ventral 

 aspects of the crura olfactoria and the metencephalon 

 into contact. The brain was secured in this condition 



upon a bed of wet cotton by pushing cotton against it, and covered with 95 per cent. 

 alcohol. 



Explanation. The names are written in full, excepting vo. for valvula. The furrow 

 between the optici is not sufficiently distinct, and the word postoptici obacures the fact 

 that the elevations so named are separated by a somewhat wide and flat valley rather than 

 by a narrow depression.. 



In the preparation, the infundibuliform mesoccelian orifice is visible through the trans- 

 parent valvula. Compare PI. Ill, Fig. 7. 



Fig. 115. The callosum after removal of the dorsal portions of the cerebellum and 

 hemispheres ; from Prep. No, 540, M. C. U. 



Objects. To show (A) the fact of the connection of the hemispheres by the callosum ; 



FIG. 114. DORSAL ASPECT OF 

 THE MESENCEPHALON, WITH 

 SOME ADJACENT PARTS; x 1. 



