THE FROG 



193 



olfactory lobes, ol, the cerebral hemispheres, ce, the thalamen- 

 cephalon, th, the optic lobes, op, the cerebellum, cb, and the 

 medulla oblongata, m. The cere- 

 bellum is very small, and the me- 

 dulla appears to be only an en- 

 larged continuation of the spinal 

 cord. In the latter there is a 

 large triangular cavity, roofed 

 over by a thin membrane con- 

 taining blood vessels (choroid 

 plexus). The cavity is called the 

 fourth ventricle, and it commu- 

 nicates with other cavities in the 

 brain. On top of the thalamen- 

 cephalon is a small body, the 

 pineal gland or the epiphysis, pi. 

 The under side of the thalamen- 

 cephalon is produced into a 

 process directed backward, the 

 infundibulum, which ends in 

 another small body, the pituitary 

 body or the hypophysis. 



The" cerebrum and thalamen- 

 cephalon together constitute the 

 forebrain, the optic lobes form 

 the mid-brain, and the cerebel- 

 lum and medulla form the hind-brain. The relative devel- 

 opment of these different parts varies widely in different 

 animals, and in the higher vertebrates the cerebrum becomes 

 much the largest part of the brain, this development reaching 

 its maximum in the human species. 



From the posterior part of the medulla the spinal cord, sp, 

 extends through the spinal column, tapering to a minute fila- 

 ment, which extends a short distance into the urostyle. The 

 brain and spinal cord are covered by two membranes, an outer 



FIG. 94. THE CENTRAL 

 NERVOUS SYSTEM 



Shown in position in the skull and 

 spinal column. 



cb, cerebellum; op, optic lobe; 



ce, cerebrum; pi, pineal body; 



m, medulla oblon- sp, spinal cord; 



gata ; th, thalamen- 



ol, olfactory sac; cephalon. 



