356 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



ends are enlarged, the former being named the genu, the latter the splenium. 

 From about the middle of the corpus callosum a band of fibers, the fornix, curves 

 ventrally. Between the fornix and the anterior half of the corpus callosum 

 stretches a thin membrane, the septum pellucidum, consisting of two leaves. If 

 the brain is sectioned exactly in the median sagittal plane, the section will pass 

 between the two leaves of the septum pellucidum; but often the whole septum is 

 left on one half; in this case a slitlike opening into a cavity, the lateral ventricle, 

 will appear on the other half between the fornix and the corpus callosum. The 

 fornix passes downward and soon turns (as the column of the fornix) into 

 the interior of the brain where it is lost to view. Immediately in front of the point 

 where it disappears is the section of a small round bundle, the anterior commissure. 

 From the anterior commissure a delicate membrane, the lamina terminalis, 

 extends ventrally to the optic chiasma. The fornix, the anterior commissure, 

 and the lamina terminalis form the anterior boundary of a deep but narrow 

 chamber, the third ventricle) which lies in the middle of the diencephalon. The 

 cavity of the third ventricle extends ventrally into the tuber cinereum and the 

 pituitary body. 



The diencephalon is the massive region extending between the fornix and 

 lamina terminalis and midbrain. It consists of three parts : a dorsal region, the 

 epithalamus; a central and lateral region, the thalamus \ and a ventral region, 

 the hypothalamus. The hypothalamus includes the optic chiasma, the tuber 

 cinereum, the mammillary body, and the hypophysis or pituitary body, all of 

 which should be identified in the section. The epithalamus includes the struc- 

 tures in the roof of the diencephalon. These are: the chorioid plexus, a thin 

 folded vascular membrane between the cerebral hemisphere and the diencephalon; 

 the pineal body, a stalked body lying in the chorioid plexus; the habenula, a 

 small mass just in front of the attachment of the pineal body to the diencephalon ; 

 and the posterior commissure, a small circular area just posterior to the habenula. 

 The thalamus constitutes the greater part of the diencephalon. On the cut 

 surface it presents a large, round mass, the intermediate mass or middle commis- 

 sure; this is not really a commissure but merely the cut median mass of the 

 thalamus. The greater part of the thalamus is concealed by the overhanging 

 cerebral hemisphere. On the smaller piece of the brain remove the cerebral 

 hemisphere and then examine the dorsal and lateral regions of the thalamus. 

 Three low elevations are present. The most dorsal and medial one is the puhinar. 

 Lateral to this and whiter in color is the lateral geniculate body. A white band, 

 the optic tract, is plainly seen ascending from the optic chiasma and terminating 

 on the lateral geniculate body. Posterior and ventral to the lateral geniculate 

 body is a smaller swelling, the medial geniculate body. Behind the geniculate 

 bodies will be recognized the corpora quadrugemina as two low hillocks. Ventral 

 to them runs the stout cerebral peduncle, the anterior part of which is crossed 

 externally by the optic tract. 



