460 ANATOMICAL TECHNOLOGY. 



intrude apparently into the cornu. In so doing, however, the endyma is carried before 

 so as to encompass the fold or the vessels, as an abdominal viscus is surrounded by the 

 peritoneum. Hence, also, the cornu is really completely enclosed by the endyma, even 

 where the nervous parietes are absent. 



The general name atriatum is given to the thickened part of the hemisphere, but the 

 cauda striati and tcenia are not represented. 



1156. Fig. 122. Transection of the brain through the diacoelia. Compare with 

 Fig' 120 ; PI. Ill, Fig. 7, 10, 13 ; PI. IV, Fig. 15. 



Objects. To show (1) that the diacoelia has a roof (diatela) independent of the velum 

 and fornix ; (2) that the dorsal surface of the thalamus is ectoccelian ; (3) that the arach- 

 noidea does not reach the bottom of the interhemispheral fissure. 



Explanation. The diacoelia is seen to be divided into a larger ventral and a smaller 

 dorsal portion by the large medicommissura. The walls (lateral parietes) of both portions 

 are the thalami. 



The dorsal surface of the thalamus is convex and covered by pia. The mesal is nearly 

 flat and covered by endyma. The rounded angle between these two surfaces presents a 

 ridge, the habena, and a farrow, the Sulcus habenae. 



The endyma leaves the thalamus along the sulcus (PI. Ill, Fig. 7), and is reflected first 

 dorsad and then mesad so as to roof in the diacoslia completely, as indicated in PI. Ill, 

 Fig 10, and by the definite form of the plaster cast shown in Fig. 120. 



In the frog and in the embryo mammal, the only parts dorsad of the diacoelia are this 

 endyma and the pia and arachnoid which envelope the entire brain, together constituting 

 a membranous diatela. But in the adult mammal, the hemispheres not only project dor- 

 sad of the diencephalon, but unite along the meson so as to form the callosum and the 

 mesal part of the fornix (lyra) ; hence these parts are dorsad of the diatela, and the latter 

 is often ignored on account of its tenuity. 



The relation of the diaplexus to the diatela is not altogether clear. In the figure only 

 one is represented. There is no evidence that it is formed by the intrusion of the pia 

 along the Sulcus habenae, and more probably it is primarily prolonged from the aula, 

 as in Menobranchus, but acquires a somewhat distinct adhesion to the diatela. 



1157. Fig. 123. Diagram of a transection of the porta (foramen of Monro). Com- 

 pare Fig. 113, 120, and PI. Ill, Fig. 18, 19. 



Object. To show that the porta is completely circumscribed, notwithstanding the ab- 

 sence of the proper nervous parietes at the dorsal end where it adjoins the rima. 



Description. The plane of section is dorso-ventral and longitudinal, practically paral- 

 lel with the meson. Regarding the porta as a short passage with an approximately trans- 

 verse direction, it is here transected. 



The larger mass is the dorso-cephalic portion of the thalamus. Cephalad and dorsad 

 of it is the fornix with its Columna, these being continuous with the corresponding half 

 of the septum lucidum. 



The dorsal surface of the thalamus is convex and covered by pia (velum), but the 

 cephalic surface is concave and covered by endyma. In man and in some cats, the bound- 

 ary between the two surfaces is indicated by a more or less distinct tubercle. In all cases 

 there is a point of reflection of the endyma cepbalad toward the fornix. 



The corresponding surfaces of the fornix are likewise pial and endymal ; there is no 

 elevation of the surface to mark the boundary between the two, but nearly opposite the 

 point of reflection of the endyma from the thalamus it is reflected also from the fornix. 



So much of the interval between the thalamus and the fornix as lies ventrad of these 

 two points of reflection is the porta. Primitively, there is reason to believe, the endyma 



