THE NERVOUS SYSTEM 425 



ciation with them the diencephalon develops in mammals a 

 pair of lateral ganglionic masses, the thalami optici, which 

 arise as thickenings of the sides of the vesicle beneath the optic 

 stalks. These are to be sharply distinguished from the lobi 

 optici (optic lobes), under which name the lateral halves of 

 the mesencephalon are usually described. 



The mesencephalon is the most conservative of the elements 

 of the brain : it develops very little that is new throughout its 

 entire history, and in Man and the other mammals, although 

 suffering little or no actual diminution in size, it becomes re- 

 duced proportionately to a very small portion of the brain 

 through the excessive growth of the surrounding parts. This 

 is made clear by the diagrams, in which the mesencephalon may 

 be followed through fishes, amphibians and reptiles with but 

 little change. Its roof and outer sides are moderately thick- 

 ened and usually divided along the mid-dorsal line by a longi- 

 tudinal groove, thus forming a bilobate organ, the corpora 

 bigemina or lobi optici. In many fishes they form a conspicu- 

 ous part of the brain which, so long as the cerebral lobes remain 

 but slightly developed, must be of great functional importance. 

 In some teleosts, for example, in which the cerebral hemi- 

 spheres are represented merely by a non-nervous membrane, 

 they furnish at least two-thirds of the dorsal surface, and thus 

 perhaps functionally replace the former. In amphibians and 

 reptiles the gradual development of the cerebral hemispheres 

 reduces the importance of the optic lobes, although in birds, 

 forms not in the direct line of human ancestry, they again reach 

 a certain prominence; thus when the enormously developed 

 corpora striata and the small and thin walls of the hemispheres 

 are taken into consideration, birds are seen to be as unique in 

 their brain development as they are in their skeleton and their 

 general form. 



In mammals the mesencephalon is to be looked for between 

 the two greatly hypertrophied elements, telencephalon and 

 metencephalon (cerebrum and cerebellum), and here the 

 bilobed organ has become transformed into one with four 

 lobes, the corpora quadrigemina. The beginning of this 



