VERTEBRATES. 



299 



narrower portions. In the brain five divisions may be 

 distinguished. Eeginning in front, these are:* (1) the 

 cerebrum, composed of right and left halves or hemi- 

 spheres, and containing in their interiors the first and 

 second ventricles; (2) the smaller 'twixt-brain, with thin 



FIG. 109. Diagram of vertebrate brain, c, cerebrum; cb, cerebellum; h, in- 

 fundibulum ; TO, medulla; o, olfactory nerve; ol, optic lobes; s, spinal cord; 

 1-4, ventricles. 



walls and enclosing the third ventricle; (3) the thick- 

 walled optic lobes; (4) the cerebellum; (5) the medulla oblon- 

 gata, the fourth ventricle being contained in cerebellum and 

 medulla. In the lower vertebrates these five regions are 

 nearly equal in size, but the higher we go in the scale the 

 larger proportionately do the cerebrum and the cerebel- 

 lum become, until in man the cerebrum weighs about 

 nine tenths of the whole brain. 



From the brain are given off, typically, twelve pairs of 

 nerves, which are spoken of both by numbers and by 

 their proper names. The majority of these are unlike 

 the spinal nerves in that they have but a single root, and 

 are correspondingly either sensory or motor. Thus the 

 first or olfactory nerve, which goes to the nose; the second 



* Other names are frequently applied to these parts, as follows: 

 Cerebrum, Prosencephalon, Parencephalon, 



Thalamencephalon, Diencephalon, 

 Mesencephalon, 

 Metencephalon, 

 Myelencephalon , 



"Twixt-brain, 

 Optic Lobes, 

 Cerebellum, 

 Medulla, 



Midbrain, 



Epencephalon, 



Metencephalon. 



