THE NERVOUS SYSTEM 



411 



animal, is somewhat enlarged and divided by two transverse 

 constrictions into three successive vesicles (Fig. 115, A), the 

 fore-, middle-, and hind-brain, or, more technically, prosen- 

 ccphalon, mesencephalon and metencephalon, each with its 

 cavity or primary ventricle. Of these the first two seem to 

 represent the very rudimentary cerebral vesicle found in Am- 

 phioxus and may be termed the archencephalon, or primordial 

 brain, while the third may be considered the anterior end of 

 the spinal cord, which becomes added to the brain at some 

 point between Amphioxus and the cyclostomes. This cerebral 

 vesicle of Amphioxus bears two rudimentary sense organs, an 

 olfactory groove and a pigment spot ; it may be more than a 

 coincidence, then, that in the higher forms the first two original 

 vesicles furnish but two pairs of nerves, olfactory and optic, 

 while the other nerves are derived from the primary third 

 vesicle. 



Of all vertebrates the cyclostomes alone possess a brain 

 which may be interpreted as still consisting of three primary 

 vesicles; in all others several modifications take place (Fig. 

 115, B). The prosencephalon becomes modified by the forma- 

 tion of two diverticula, which are thrown out from the sides 

 and grow anteriorly, often reaching a point considerably be- 

 yond the anterior limits of the primary tube. These are the 

 two lobes of the telencephalon (the cerebral hemispheres of 

 the higher vertebrates), in distinction from which the un- 

 paired remainder is designated as the diencephalon. 



Internally the primary first ventricle becomes divided into 

 the two lateral ventricles and the one naturally denominated 

 the third; the latter communicates with the two first through 

 a passage which is inclined to become narrow, the foramen in- 

 terventriculare [foramen of Monro~\. 



The mesencephalon is the most conservative of the primary 

 vesicles, and other than a lateral expansion which sometimes 

 forms a pair of prominent optic lobes, suffers no marked 

 change. Its ventricle is often large and obvious, but has re- 

 ceived no special name or number. 



The third primary vesicle, the metencephalon, shows a 



