DEMONSTRATION OF THE PORTA. 457 



about 2 mm. of the end of the Ldbus. This canal is the reduced 

 representative of the rhinoccelia, which is quite large in most lower 

 vertebrates and in many mammals, but nearly or quite obliterated 

 in man. It is most easily traced by means of a beaded bristle. If 

 the bristle is gently moved to and fro, enough of the coloring matter 

 of the bead will adhere to the sides of the canal to make its recogni- 

 tion more easy as it is exposed, either by removing very thin slices 

 or by following it up with the tracer. The diameter of the canal is 

 about .5 mm., but it is usually expanded a little at its extremity. 



§ 1151. Demonstration of the Porta. — Recall the position of the 

 aula upon the meson. Hold the brain in alcohol or water so that 

 only the surface cephalad of the dorso-ventral incision projects 

 above the surface; then blow toward the aula from between the 

 striatum and the mesal wall of the prsecornu. The escape of bub- 

 bles of air will demonstrate the connection, through the porta., of 

 the aula with the procoelia ; see Fig. 120 and PL lY, Fig. 16. 



§ 1152. Exposure of the PrcEcommissura. — Remove the cephalic 

 end of the brain by an incision at about the middle of the striatum, 

 and note, on the cut surface, the alternation of alba and cinerea on 

 account of which the name was applied. Remove other slices, cut- 

 ting a little obliquely, latero-cephalad, and note the increasing dis- 

 tinctness of an oblong white area, the oblique section of a fibrous 

 fascicfiilus — ^the praecommissura — which unites the striata and Lobi 

 olfactorii across the meson, and wliich has been observed already in 

 the examination of the mesal surface ; see also PL IV, Fig. 14. 



Fig. 119. — Plaster casts of the medicornua, inverted. 



Fijr. 130. — Plaster casts of the diacoelia, aula, right porta, and part of the right pro- 

 CCElia of the sheep, inverted. 



Fig. 121. — Diagram of a transection of the left medicornu. 

 Fig. 122. — Diagram of a transection of the diaccelia. 

 Fig. 123. — Diagram of a transection of the porta. 



§ 1153. Fig. 119. — Plaster casts of the medicornua, inverted; xl.5. Compare with 

 PI. IV, Fig. 1.5. 



Objects, — To show (1) the shape and extreme curvature of these parts of the pro- 

 ccelise ; (3) that they are completely circumscribed, and do not open by a "fissure of 

 Bichat " upon the surface of the hemisphere. 



Preparation. — The plaster was injected through the Foramen infundibuli while the 

 brain was supported by the calva (§ 113.5). After resting undisturbed for an hour, the 

 brain substance was carefully torn and cut sufficiently to extricate the casts. 



Explanation. — The ventral ends are thinner and somewhat expanded, excepting the 

 extremities, which are decidedly contracted. This contracted finger-like portion is wholly 

 enclosed by true nervous parietes ; all the rest is bounded along the concave (cephalic) 



