106 VERTEBRATE ZOOLOGY 



The Brain and the Spinal Cord. In order to expose these organs, 

 remove the skin and muscles from the back of the head and trunk. 

 Find the juncture of the skull with the backbone. By bending the 

 head slightly down, a space about an eighth of an inch long, which 

 is covered by a dark-colored membrane, may be made to appear 

 between the skull and the backbone. With a needle very carefully 

 remove this membrane. Beneath will be seen the white brain ; this 

 must not be injured. Introduce one blade of the scissors into the 

 skull through the opening and make a cut along the side of the 

 skull between the eyes. Make a similar cut along the other side, 

 and with the forceps lift off the roof of the skull, thus exposing the 

 brain. Similarly cut through the two sides of the neural canal, 

 which contains the spinal cord, and expose it. 



Carefully remove the dark membrane which covers the brain, 

 and observe the five regions which appear in a dorsal view: the 

 cerebrum ; the diencephalon ; the optic lobes, or midbrain ; the 

 cerebellum ; and the medulla oblongata. 



The brain and spinal cord are hollow structures. A delicate 

 canal, called the central canal, runs through the center of the cord ; 

 in the brain this canal widens out into a number of ventricles. 



The anterior and largest region of the brain is the cerebrum. 

 It is made up of the two lateral hemispheres, which are separated 

 from each other by the sagittal* fissure. The anterior ends of the 

 hemispheres are fused and form the olfactory lobe, from the ante- 

 rior end of which the two olfactory nerves pass to the nose. 



Back of the cerebrum is the inconspicuous diencephalon, and 

 behind that are the paired optic lobes, or midbrain. In the roof 

 of the diencephalon will be seen, with the aid of the lens, several 

 delicate structures. Near the center of it arises a threadlike pro- 

 jection called the pineal body, or epiphysis, which extends forward 

 over the diencephalon. In an early period of the larval life of the 

 frog the epiphysis extends through the skull to the skin on the top 

 of the head between the eyes, where it joins the brown spot known 

 as the frontal organ, which is the rudiment of the pineal eye ; this 

 connection is lost before the animal becomes adult. 



In front of the epiphysis and between the hinder ends of the 

 hemispheres is the much larger paraphysis, a dark-colored, vascu- 



