120 HUMAN PHYSIOLOGY 



bones of the skull. We thus speak of the frontal or 

 forehead region of the brain (Fig 34, /.c.), the parietal 

 or side region, the temporal region at the side of 

 the brow, and the occipital region (o.L), or that at 

 the back of the head. 



BRAIN CELLS. A section of the cerebrum shows 

 us that its outer layer consists of grey matter com- 

 posed of brain cells, this grey matter following and 

 dipping into the convolutions thereby affording a 

 larger amount of cell material than would be the case 

 in a smooth brain. Groups of brain cells are found 

 in the other parts of the brain. In the cerebrum the 

 grey matter is the external layer, whilst the white 

 matter, composed of nerve fibres which carry 

 messages to and from the brain centres, forms the 

 inner portion of the cerebrum. It may be said that 

 the most important brain cells are those of this 

 outer layer of the great brain. They must exist in 

 millions, one estimate giving over 800,000,000 as an 

 approximate number. The cerebellum when cut in 

 a section is seen to have its brain cells arranged in 

 different fashion. Externally it presents a layered 

 appearance (Fig. 32), and in section the white matter 

 forms a tree-like pattern to which the fanciful name 

 of arbor vit<z, or " tree of life " has been given. The 

 white matter of the brain, consisting as we have seen 

 of fibres, is arranged in definite bands or tracts, and 

 thus, like the telephone system of a great factory, 

 provides for inter -communication between the 

 various groups of brain cells, as well as affording 

 communication between brain and body. 



BRAIN FUNCTIONS. The brain in respect of the 

 functions it discharges might be not inaptly com- 

 pared to a three-storied warehouse. The lower 

 storey of the warehouse we may suppose is given 



