BRAIN AND NERVE 



97 



The Human Brain. The human brain is represented in a very 

 diagrammatic way in Fig. 27. 



It is supposed that the whole organ has been cut through in a 

 mid-vertical plane, and a number of details that are rather 

 difficult to understand have been omitted from the diagram. 

 The parts actually cut through are cross-hatched, and we see the 

 central canal of the spinal cord widening out to form the " fourth 

 ventricle " of the anatomists that is, the primary hind-brain 

 vesicle. Between this and the " third ventricle " that is, the 



I Olfactory 



Sbinal 

 ' cord. 



K.Optic(eyes) 



JJf Oculomotor 



(muscles ofeyesj 



)Y,Trochlea.r 



( M uscles of eyes) 



-Yf.Abducens 

 , (muscles of eyes) 



Y- Trijgeminal 

 (facet-Jan*) 



yM Auditory 

 (ear) 



facial 



.ossof)ha.ryn$ eal 

 (Mouth ( pharynx., 



X. Vagus 

 (Gills, heart**) 

 ifia/ nerves 



FIG. 26.- THE BRAIN OF A FISH (COD) SEEN FROM THE UPPER OR 

 DORSAL SURFACE. 



primary fore-brain vesicle is a narrow passage which is the 

 " aqueduct of Sylvius," and this represents the primary mid- 

 brain vesicle. The floor, sides, and roof of the hind-brain vesicle 

 are enormously thickened to form the cerebellum and the 

 peduncles which connect it with the other parts of the brain. 

 Similarly the roof, sides, and floor of the mid-brain vesicle have 

 become thickened to form the corpora quadrigemina, and the 

 great crura, or peduncles of the cerebrum, and some other parts. 

 The roof and floor of the " third ventricle " remain thin, but the 



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