152 THE NERVOUS SYSTEM. 



shape by pressure; it therefore requires to be placed in a 

 well-protected 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 com- 

 pletely, disorganize that structure. Within the skull, the 

 brain is enveloped by certain membranes, which at once 

 protect it from friction, and furnish it with a supply of 

 nutrient vessels ; they are called the arachnoid, or "spiders 

 web," the dura mater and the pia mater, or the "tough" 

 and "delicate coverings." The supply of blood sent to 

 the 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 organs, and occupies the en- 

 tire upper and front portion of the skull. It is almost com- 

 pletely bisected, 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, 

 however, is composed almost wholly of white substance, or 

 nerve-fibres. 



9. The surface of the cerebrum is divided by a consid- 

 erable number of tortuous and irregular furrows, about 

 an inch deep,' into "convolutions," as shown in Fig. 41. 

 Into these furrows the gray matter of the surface is ex- 

 tended, and, in this manner, its quantity is vastly in- 

 creased. The extent of the entire surface of the brain, 



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

 The interior ? 



9. The surface of the cerebrum, how marked ? The gray matter of the surface ? 

 Extent of the entire brain surface ? Source of nervous power ? What further ? 



