290 STRUCTURE OF THE VERTEBRATES 



The development of the neopallium causes several important 

 changes. One of the first forebrain centers to become separated 

 from the olfactory function is the hippocainpus, located as an 

 important nucleus on the side of the telencephalon in the lower 

 groups including the reptiles. In the mammals this region be- 

 comes infolded and covered by the cerebral hemispheres. The 

 corpus striatum enlarges and is also pushed deeper into the brain 

 tissue, along with smaller nuclei. With the continued growth 

 of the cortex in the higher mammals the shallow creases multiply 

 in number, and certain ones become deep fissures separating 

 the cerebrum into fairly well-marked lobes. The four major 

 lobes are the frontal forming the dorsal and anterior portion; 

 the parietal posterior to the former; and the occipital as the 

 most posterior portion of the cortex. The outer ventral region 

 is the temporal lobe. The names are most applicable to the 

 human, in which the lobes correspond generally to the skull 

 bone of the same region. 



DiENCEPHALON. The diencephalon is a relatively small region 

 between the telencephalon and the midbrain. The ventral region 

 is invaded by the thalamic nuclei, important points of transfer 

 for impulses from the brain to the cord. The diencephalon con- 

 tains the large third ventricle, the older anatomists having num- 

 bered the right and left ventricles of the cerebral hemispheres 

 one and two. The dorsal wall of the diencephalon is a thin mem- 

 brane on which lies the choroid plexus of blood vessels. In most 

 vertebrates the plexus sinks into the ventricular space, osmosis 

 giving a constant supply of nutrient fluid to the ventricular 

 spaces. This is the most important blood plexus of the brain. 



From the diencephalon arise two evaginations, one dorsal and 

 one ventral, which play an important part in growth and de- 

 velopment. The dorsal epiphysis is a glandular tube in the cyclo- 

 stomes and fish. From this structure, or from nerve cells which 

 grow out with it, a third (or parietal) eye developed in the 

 earliest amphibia. A dorsal eye was of selective value to mud- 

 living animals with wide flat heads, and the structure was car- 

 ried over to the primitive reptiles. Living amphibia have little 

 evidence of the structure, but a number of reptile embryos de- 

 velop a dorsal optic vesicle which soon degenerates. The more 

 primitive reptilian groups develop not only an optic vesicle, but 



