TRANSECTIOX OF THE MEDICORNU. 459 



border by the rima, where the continuity of the endyma reflected upon the proplexus, and 

 its adhesion along the fimbria and in the Sulcus limitans prevented the escape of the 

 injected material. 



1154. Fig. 120. Plaster casts of the diacoelia (dorsal portion), aula, and right 

 porta and praecornu of the sheep, inverted ; x 1.5. Compare with Fig. 113, 122 ; PI. Ill, 

 Fig. 7, 10 ; PI. IV, Fig. 16, 18, 19, 



Objects. To show that (1) the porta permits the passage of injection mass from the 

 aula into the proccelia ; (2) the aula, porta and diacoelia are completely circumscribed ; 

 (3) the porta is a passage of some length, and strongly compressed. 



Preparation. The plaster was injected through the Foramen infundibuli ( 1125). 

 Notwithstanding all possible caution in disengaging the casts, a fracture occurred just 

 between the porta and the aula. In mounting, a slight interval was left between the 

 parts. The left porta and procoelia are not included in the figure. The ridge on the ven- 

 tral aspect of the cast of the diacoelia corresponds with the dorsal contour of the medicom- 

 missure. The cast of the praecornu is quite thin, and its lateral aspect presents a sharp 

 longitudinal depression corresponding with the ridge upon the striatum (PI. IV, Fig. 16). 



Qualification. The three following figures (121-123) are dia- 

 grammatic representations of our present understanding of the 

 relations of the plexuses to the membranous parietes of the porta, 

 diacoelia and medicornu. 



It is probable that microscopic sections will be needed in order to elucidate certain 

 points, especially the question whether the proplexus and portiplexus are formed by the 

 intrusion of the entire pia or only of vessels therefrom ( 1081). We have thought best 

 to represent the former view provisionally. 



Admitting that in respect to this detail we may be in error, we wish to insist upon the 

 more essential point, viz., that these cavities are really and completely circumscribed by the 

 continuity of the endyma reflected from the borders of the nervous parietes upon the 

 plexuses. 



1155. Fig. 121. Diagram of a transection of the left medicornu. Compare with 

 Fig. 113, 119 ; PI. Ill, Fig. 11 ; PI. IV, Fig. 14, 15 ; also with Duval (2, PI. I, Fig. 2). 



Object. To show that the medicornu is completely circumscribed, although the proper 

 nervous parietes are absent from the porta to near its tip, constituting the rima. 



Explanation. The figure is based upon Fig. 113, but the cornu is disproportionally 

 enlarged, as are also the distances between the hemisphere and the mesencephalon and 

 diencephalon. The membranes and the proplexus are also introduced. 



It should be remembered that this represents a transection of the length of the medi- 

 cornu, although, from the fact that the plane of section is horizontal with respect to the 

 brain as a whole, the cavity closely resembles a longitudinal exposure of the postcornu of 

 man and the monkeys, which does not exist in the cat. 



The ectal surfaces of the hemisphere and of the epencephalon, mesencephalon, post- 

 geniculatum and praegeniculatum are covered by pia. The cornu is lined by endyma. 



In the frog and presumably in the embryo of the cat, the entire length of the medi- 

 cornu has complete nervous parietes, as has the tip of the eornu in the cat and man. The 

 endyma and the pia would then be wholly separated. 



But the abrogation of the nervous continuity between the border of the fimbria and 

 the Sulcus limitans permits the vessels of the pia, or perhaps a fold of the pia itself, to 



