646 



THE CENTRAL NERVOUS SYSTEM 



[CH. XL1V. 



Figs. 397 and 398 represent a diagrammatic view of a vertebrate 

 brain; the attachments of the pineal gland, pituitary body, and 

 olfactory (I) and optic (II) outgrowths are also shown. 



Fro. 398. Longitudinal and vertical diagrammatic section of a vertebrate brain. Letters as before. 

 I'V, poris Varolii. Lamina terminalis is represented by the strong black line joining Pn and Py. 

 (Huxley.) 



These diagrams might serve very well for the brain of an adult 

 selachian fish, such as a shark. The olfactory bulb is, however, very 

 much larger, and the fore-brain smaller. In the shark, smell is the 

 all-important sense ; the olfactory nerves, which originate from the 

 olfactory bulb, spread out over an immense area many square feet in 

 size (12 to 13 square feet in a shark 25 feet long). Behind the 

 olfactory bulb is another focus of grey matter, called by Edinger the 

 parolfactory lobe, connected to the fifth nerve, the sensory nerve of 

 the mouth. No doubt the oral sense, as it is termed, is important in 

 the pursuit and capture of prey ; it always is in animals who catch 

 food with the mouth. One sees it highly developed in animals with 

 prehensile tongues, and bristles or whiskers on the lips ; also in birds, 

 with their sensitive beaks and bills. 



Returning, however, to the shark, we find the cerebellum is large, 

 as it is in all powerful swimmers and flyers, but the cerebrum in the 

 strict sense is absent ; there are no hemispheres and no grey cortex ; 

 the fore-brain consists of little else but the corpora striata. 



The cerebral hemispheres are later growths superimposed upon 

 this primitive brain, and in the animal series one notes the progres- 

 sive development of the cerebrum in relation to function and adapta- 

 tion to environment. 



The primitive brain as exemplified in that of the shark is common 

 to all animals up to man, and is termed by Edinger the Pala- 

 encephalon or old brain. The cerebral hemispheres constitute what 

 he terms the Neo-encephalon or new brain. 



The neo-encephalon is specially characterised by the possession 

 of a grey cortex, and this is the seat of the psychical or mental 

 processes termed volition and sensation. The first part of the cortex 

 to appear in development is called the archipallium (or old cortex), 



