THE NERVOUS SYSTEM 



211 



B 



tected position, such, as is afforded by the skull, or cranium, 

 which is strong without being cumbrous. In the course of an 

 ordinary lifetime, this bony box sustains many blows with little 

 inconvenience ; while, if they fell directly upon the brain, they 

 would at once, and completely, disorganize that structure. 

 Within the skull, the brain is enveloped by certain mem- 

 branes, which at once protect it from friction and furnish it 

 with a supply of nutrient vessels ; they are called the arach- 

 noid, or " spider's web," the dura mater, and the pia mater, or 

 the "tough" and "delicate coverings." The supply of blood 

 sent to th-e brain is very liberal, amounting to one-fifth of all 

 that the entire body possesses. The brain of man is heavier 

 than that of any other 

 animal, except the ele- 

 phant and whale. 



8. The Cerebrum. - 



The brain proper, or 

 cerebrum, is the largest 

 of the intracranial or- 

 gans, and occupies the 

 entire upper and front 

 portion of the skull. It 

 is almost completely bi- 

 sected by a fissure, or 

 cleft, running through it 

 lengthwise, into two equal 

 parts called hemispheres. 

 The exterior of these 

 hemispheres is gray in 

 color, consisting chiefly 

 of nerve-cells,- arranged 

 so as to form a layer of 

 gray matter one-fifth of an inch in thickness, and is abundantly 

 supplied with blood-vessels. The interior of the brain, how- 

 ever, is composed almost wholly of white substance, or nerve- 

 fibres. 



FIG. 55. UPPER SURFACE OP THE CEREBRUM 



A, Longitudinal Fissure 



B, The Hemispheres 



8. Size of the brain proper ? How divided ? The exterior of the hemispheres ? The 

 interior ? 



