706 EVOLUTION OF ANIMALS Part V 



and backward through a small hole (foramen of Munro) which opens into 

 the third ventricle. They are finally continuous with the central canal of the 

 cord. Thus, all of them are open to the circulation of the cerebrospinal fluid 

 and there is a serious disturbance if the passage becomes closed. 



In frogs, the nerve cells of the cerebral lobes seem to function mostly in the 

 conduction of nerve impulses from the olfactory lobes to a more posterior 

 region (thalamus). If the olfactory and cerebral lobes are removed the frog 

 sits, jumps, and eats as usual, a contrast to the result even of a minor injury 

 to the cerebrum of a mammal. 



BETWEEN BRAIN, DiENCEPHALON (oR thalamencephalon) . Directly be- 

 hind the cerebral lobes is a folded membrane, the anterior choroid plexus that 

 forms the roof of the median third ventricle. Its large blood supply is important 

 to the brain which, like the human brain, has work to do. The pineal stalk, a 

 delicate stemlike process, reaches up to the cranium and, in the skin above it, 

 is marked by the brow spot. These structures are remains of a third eye present 

 in the ancestral amphibians. The optic nerves from the eyes reach the dien- 

 cephalon just below the third ventricle. All the processes from nerve cells in 

 the right eye cross over to the left side of the brain, and those from the left 

 eye cross to the right side thus forming the optic cross or chiasma. There are 

 theories regarding it but the reason for this crossing is not known; in higher 

 vertebrates, it is only partial (Chap. 17). The sides of the diencephalon are 

 thickened and form the thalami over which the cell processes of the optic 

 nerves spread out fan-wise before entering the optic lobes. Behind the optic 

 chiasma, the floor of the brain projects downward toward the mouth and is 

 joined to a little mass of glandular cells originating from the wall of the 

 mouth (Fig. 34.22). This compound structure is the pituitary gland or 

 hypophysis (Chap. 15). 



After the diencephalon is removed with the cerebral lobes, a frog seldom 

 moves voluntarily. It is completely blind because its optic nerves have been 

 cut. When placed on a tilted board it will not climb like the frog from which 

 only the cerebral lobes are removed. Neither can it keep its balance on the 

 edge of the board. Placed on a rotating disk it will try to adjust itself by turning 

 its head opposite to the direction of rotation. 



MIDBRAIN OR MESENCEPHALON. In fishcs and amphibians, this short section 

 of the brain stem is expanded on its dorsal side into prominent optic lobes. On 

 its under surface are two ridges, the crura cerebri, literally the legs of the 

 cerebral hemispheres. These are composed of cell processes extending from the 

 medulla to the cerebral lobes. Cavities in the optic lobes communicate with 

 the slender central passage connecting the third and fourth ventricles (Fig. 

 34.22). 



In lower vertebrates, the midbrain is a coordinating center and impulses 

 enter it through the nerves from the eyes, ears, and certain other parts of the 



