THE NERVOUS SYSTEM 43 



point two inches behind the eyes. Cut away the cartilage 

 and expose the brain. The white strands are cranial nerves, 

 which pass through the cartilage and should not be injured. 

 The brain is situated in a cavity in the chondocranium 

 and is covered by the primitive meninx, the membrane in 

 which the blood vessels of the brain run. This membrane 

 is connected here and there to the walls of the cranial cavity 

 by strands of tissue. The space between is filled with fluid 

 in life. 



a. The Telencephalon. At the anterior corner of the 

 main brain mass is the elongated olfactory bulb. Cut away 

 the cartilage and expose the bulb. It gives off a very small 

 olfactory nerve forward to the olfactory sacs. The bulb 

 is connected by the stock, the olfactory tract, to the en- 

 larged olfactory lobes which are a part of the main mass 

 of the brain. Medial to these lobes and separated only by 

 a slight groove are the cerebral hemispheres. 



b. The Diencephalon or Thalamencephalon. Posterior 

 to the hemispheres is the depressed diencephalon. The thin 

 roof of this consists of the choroid plexus, blood vessels 

 of the third ventricle. The diencephalon is the center for 

 the coordination of sensations and is the chief controlling 

 portion of the brain. The optic nerve passes from the orbit 

 towards its ventral surface. 



c. The Mesencephalon or Midbrain. The mesencephalon 

 is posterior to the diencephalon and consists largely of the 

 optic lobes (corpora bigemina), which are centers for the 

 auditory, visual and skin sensations. The trochlear (fourth 

 cranial) arises from the posterior edges of the optic lobes 

 and runs forward to the muscles of the eye. The oculomo- 

 tor (third cranial) passes to the orbit from the ventral side 

 of the mesencephalon. 



d. The Metencephalon or Cerebellum. Posterior to the 

 optic lobes and overhanging them is a large cerebellum. 



