232 



THE NERVOUS SYSTEM 



matter of fact, however, the brain itself is really but an 

 enlarged and specialized development of the end of the 

 spinal cord and is found fully developed only in the 

 higher animals. The spinal cord may therefore be con- 

 sidered as projecting into the skull and there forming 

 the axis from which the various parts of the brain arise. 



Cerebellu m. 



Springing from this Olfactory lobe- 



axis and covering the 

 spinal bulb is a large 



gray mass, the cere- 



Midbrain 



(optic lobe) 

 Hindbrain 

 (cerebellum) 

 V 



VIII 

 X 



Afterbrain (spinal 

 XI 



Forebrai 

 (hemispheres) 



'helium, the surface 

 of which is broken 

 up into many ridges. 

 When cut open, it is 

 seen to consist of a 

 thin layer of gray 

 material, which con- 

 tains nerve cells, and 

 a much larger inner 

 white portion made 

 up of nerve fibres in 

 their fatty sheaths. 



Cerebrum. Arising from the same axis, a short dis- 

 tance in front of the cerebellum, are two large masses 

 of soft gray tissue, the hemispheres, which together con- 

 stitute the largest part of the brain, the cerebrum. 

 When the cerebrum is cut through, the outer gray por- 

 tion, or cortex, is found to be about one-eighth of an inch 

 thick and to be made up of nerve cells. Beneath this is 

 white matter made up of nerve fibres in their fatty sheaths. 



Brain convolutions. In the case of the higher ani- 

 mals, the extent of the gray matter is greatly increased 

 by furrows, which dip deeply into the brain substance 



Spinal cord 



FIG. 127. Brain of a pigeon, with the cranial 

 nerves indicated by numerals. (After 

 Wiedersheim.) 



