136 THE CEREBRUM. 



continuous with the pia on the posterior surface of the mid-brain 

 and cerebellum. Each border constitutes the chorioid plexus 

 of the lateral ventricle, and is seen (through the epithelium) in 

 the floor of its central part. The median part of the inferior 

 lamina of the chorioid tela invests the roof epithelium of the third 

 ventricle, and, the lateral portion covers the medial half of the 

 upper surface of each thalamus. This layer forms the two chorioid 

 plexuses of the third ventricle, which depend from its median por- 

 tion. Between the inferior and superior laminae is enclosed some 

 connective tissue through which the internal cerebral veins run 

 backward to the base of the tela; there they unite and form the 

 great cerebral vein (Galeni). 



Anterior Wall. The anterior commissure (commissura an- 

 terior cerebri, Figs. 29, 40 and 73) is a very distinct round bundle 

 of white fibers about an eighth of an inch in diameter. It is seen 

 in the anterior wall of the third ventricle supporting the roof 

 epithelium. The epithelium there bends down between the 

 columnae of the fornix and invests the ventricular surface of the 

 commissure. The columnse of the fornix and the commissure 

 bound the recessus triangularis, in which the roof and anterior 

 wall of the third ventricle meet. The anterior commissure rests 

 upon the upper extremity of the lamina terminalis, between the 

 columnae fornicis, behind, and the lamina rostralis of the corpus 

 callosum, in front. With the last two structures it is developed 

 in the lamina terminalis. It is the most important connecting 

 link between the hemispheres in vertebrates without a corpus 

 callosum (all below mammals). Bending sharply backward 

 in the cerebral hemisphere the' anterior commissure pierces the 

 inferior part of the globus pallidus and then radiates toward the 

 cortex, some of its fibers entering the external capsule. It con- 

 tains two groups of fibers: (i) The anterior group, which is the 

 commissure of the rhinencephalon, called the pars olfactoria; 

 and (2) the posterior group, the pars occipito-temporalis. The 

 pars olfactoria probably contains two fasciculi: (a) A commis- 

 sural bundle which bends sharply downward to the olfactory 

 tracts and joins the two olfactory bulbs together; and (b) a 

 decussating bundle which joins each olfactory bulb to the opposite 



