4i6 HISTORY OF THE HUMAN BODY 



three chorioid plexuses, one for each telencephalic lobe and one 

 for the diencephalon (third ventricle). The thickening in the 

 floor forms the area to be known later as the corpus striatum; 

 that of the roof and sides is the potential cerebrum. The two 

 telencephala thus represent the cerebral hemispheres together 

 with the corpora striata ; their cavities are the lateral ventricles 

 in which lie the two plexus chorioides, the entrance of which 

 into the ventricles is effected through the interventricular 

 foramina. The anterior portion of each telencephalon forms 

 an extensive olfactory lobe (rhinencephalon), which is here 

 voluminous and stalked. This latter portion is in reality noth- 

 ing less than the " olfactory nerve," which, when stalked as 

 here, and especially when prolonged, as in some lizards and in 

 birds, gives the appearance of a true cranial nerve. It is here 

 seen not^to be a genuine cranial nerve, but an element of the 

 brain. , 



The teleosts and ganoids show a unique development of this 

 part; the entire roof and sides remain membranous, but the 

 corpora striata are enormously developed. Since the mem- 

 branous portion, which is here called the pallium, or mantle, 

 is absolutely transparent and extremely delicate, it is usually 

 lost in dissection, or if retained, seems of no importance ; and 

 as the corpora striata are very large and convex, they seem to 

 the casual observer to be the true cerebral hemispheres. The 

 rhinencephala are well developed and appear as the direct con- 

 tinuation of these latter parts. 



The telencephalon of amphibians and reptiles is not unlike 

 that of the selachians (dog-fish), of which it seems a direct 

 descendant. The rhinencephalon is proportionately smaller, 

 although in many lizards it becomes greatly extended, in adap- 

 tation to the prolonged snout. 



In birds there is again, as in teleosts, an enormous develop- 

 ment of the corpora striata, which makes up the bulk of the 

 cerebrum, although the roof and sides have some thickness and 

 are not reduced to the condition of a pallium. In the mammals 

 the telencephalon reaches its highest development, when it usu- 

 ally greatly exceeds in bulk the remainder of the brain. This 



