50 F. W. THYNG 



NERVOUS SYSTEM 

 Brain 



The surface of the brain is represented from the left in plate 

 2, and from the right in plate 3. The brain is shown in median 

 sagittal section from the right side in plates 1 and 4, By the 

 cephalic flexure (Flexxeph.) which occurs in the mesencephalon 

 (Mesen.) the fore-brain is bent at an acute angle to the hind- 

 brain. The cervical flexure (Flexxerv.) is nearly a right angle. 



Telencephalon. The telencephalon {Telen.) is sharply marked 

 off dorso-laterally from the diencephalon {Dien.) by a deep ex- 

 ternal groove and a corresponding internal ridge or fold, the 

 velum transversum. This fold forms the caudal boundary of the 

 'interventricular foramen {Fo.int.). From this fold the line of 

 demarcation extends ventrally just behind the optic evaginations 

 to the postoptic recess (R.po.-op.). The telencephalon thus 

 bounded contains a median cavity, the anterior part of third 

 ventricle. The latter communicates with the lateral ventricles 

 by comparatively large crescentic openings, the interventricular 

 foramina (Fo.int.). The median cavity is bounded anteriorly by 

 the lamina terminalis. The large oval hemispheres* represent 

 the dorsal zones of the telencephalon. Each protrudes consider- 

 ably beyond the lamina terminalis and presents orally two inter- 

 nal depressions with corresponding external swellings. These are 

 the developing anterior and posterior olfactory lobes [Rhin.). 

 Caudal and dorsal to the olfactory area the wall of each hemi- 

 sphere is much thickened to form the corpus striatum (C.str.) which 

 appears as a prominent swelling on the ventral surface in front 

 of the praeoptic recess [R.p.-op.). Externally the position of the 

 corpus striatum is marked by a shallow depression, the develop- 

 ing lateral fossa. The praeoptic recess is a slight groove passing 

 transversely across the lamina terminalis into the optic stalk of 

 either side. It represents the cephalic extension of the sulcus 

 limitans, His ('92), Johnston ('09) and others. The ventral 

 zones of the telencephalon comprise the area between the prae- 

 optic and the postoptic recesses, which marks the place of later 

 development of the optic chiasma. In regard to the area of 



