lyg PHYLUM CHORDATA 



Carefully remove the dark membrane, the pia mater, which 

 covers the brain, and observe its five regions : the cerebrum, 

 the diencephalon, the optic lobes, 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 spaces 

 which are called the ventricles. 



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

 It is made up of the two lateral hemispheres. The anterior ends 

 of the hemispheres are fused and form the olfactory lobe, from the 

 anterior 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 

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

 jection called the pineal body, which extends forward over the 

 diencephalon. In an early period of the larval hfe of the frog the 

 pineal body 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. 



Back of the optic lobes and separated from them by a deep 

 groove is a narrow, transverse ridge, the cerebellum, and back of 

 that is the medulla oblongata, which is continuous with the spinal 

 cord. The dorsal wall of the medulla is a dark-colored, vascular 

 membrane called the posterior choroid plexus, beneath which is 

 the fourth ventricle of the brain. The triangular, depressed area 

 which these structures form is called the fossa rhomboidalis. 



The spinal cord is the portion of the central nervous system 

 which lies in the neural canal of the spinal column. It is a thick 

 white band, oval in cross section, from which the paired spinal 

 nerves spring. At two points it is swollen : (i) where the spinal 

 nerves which form the brachial plexus, and (2) where those form- 

 ing the sciatic plexus, respectively, leave it. The hinder end of 

 the cord tapers rapidly until it becomes a fine thread which ex- 

 tends into the urostvle. 



