1'IIVSIOLOGY FOR BEGINNERS 



The brain is composed of white and grey matter of the same 

 nature as the white and grey matter of the spinal cord. The 

 arrangement of the white and grey matter varies in different 

 parts of the brain, but is always the same in corresponding 

 parts on the two sides. In the spinal bulb the arrangement is 

 like the arrangement in the cord, except that there is more 

 grey matter relatively to the white. In each cerebral hemi- 

 sphere the grey matter forms a layer at the surface called 

 the cortex, and the white matter which this layer covers 

 consists chiefly of nerve fibres which are passing inwards from 



this surface grey matter 

 all round. These fibres 

 are largely collected 

 into bundles at the 

 lowest part of the hemi- 

 sphere, forming the crura 

 cerebri and other con- 

 nections of the hemi- 

 spheres with other parts 

 of the brain. 

 The Cranial Nerves. 

 Twelve pairsof nerves, 

 called the cranial nerves, 

 arise from the brain, 

 one nerve of each pair 

 arising from one side, 

 and the other from the 

 corresponding part on 

 the other side of the 

 brain. 



The first pair. The 

 olfactory nerve, or 

 nerve of smell, arises 



from the front of each cerebral hemisphere. 

 The second pair. The optic nerve, or nerve of sight. 

 The third pair. The motor nerve going to four of the 



muscles which move the eyeball. 



The fourth pair. Motor nerve, going to one of the muscles 

 .(the superior oblique muscle) which moves the eyeball. 

 The fifth pair : called the trigeminal nerve, because it 



FIG. 87. rPiagram showing the origin of the 



cranial nerves on the left side. 

 H, Cerebral hemisphere ; C.S, corpus striatum ; 

 Th, optic thalamus; C.Q, corpora quadri- 

 gemina ; Cb, cerebellum ; M, medulla ob- 

 longata ; I. to XII., the cranial nerves ; Sp. i, 

 Sp. 2, the first and second spinal nerves. 



