BRAIN 



891 



BRAIN 



ture apparatus; the brain of Napoleon was so 

 gigantic that it required nearly all of Europe 

 to move it to Saint Helena that it might no 

 longer demolish thrones and crumble empires. 

 The human brain is relatively larger than that 

 of the lower animals. The average weight of 

 the brain for men is about three pounds; for 

 women, it is a little less. 



Structure. The brain fills all the cavity 

 enclosed by the skull and commonly known as 

 the cranium. It is divided into four parts, the 

 cerebrum, or large brain; the cerebellum, or 

 small brain; the ports, and the medulla ob- 

 longata, which forms the upper end of the 

 spinal cord. 



Membranes. The brain is surrounded by the 

 membranes. The dura mater is strong and 

 tough. It forms the lining of the skull and the 

 partitions which separate the hemispheres of 

 the cerebrum and the cerebrum from the cere- 

 bellum. The pia mater is more delicate and 

 lies next the brain. Between these is the arach- 

 noid (spider's web), which is formed from the 

 inner layer of the dura mater and the outer 

 layer of the pia mater. 



Cavities. Under the cerebrum, and almost 

 in the center of the head, are two cavities 

 known as lateral ventricles. They are sepa- 

 rated by a thin membrane and connect with 



UPPER SURFACE OF CEREBRUM 



THE BASE OF THE BRAIN 



TWO VIEWS OP THE BRAIN 



(a) Anterior lobe of the cerebrum (e) Optic tract 



(b) Olfactory nerve (h-m) Hemispheres of the cerebellum 



(c) Portion of posterior lobe (r) Pons Variolii 



(d) Optic chasm 



Gray and White Matter. Brain substance is 

 of two kinds, gray matter and white matter. 

 The gray matter is on the outside, and forms 

 that portion of the brain which is the center 

 of all mental and voluntary physical acts. It 

 follows the folds, or convolutions, in a thin 

 layer, composed of minute cells, so small they 

 cannot be seen except by a powerful micro- 

 scope. It is estimated that the gray matter 

 of the cerebrum contains more than nine billion 

 of these cells. The white matter is more com- 

 pact and forms the greater portion of the brain. 

 It unites with the membrane which holds the 

 parts of the brain together, and with the 

 nerves. Extending from the ^ray matter 

 through the white to the various centers in 

 the brain are numberless nerve fibers, 'which 

 serve to connect these centers with each other. 



the third ventricle, which is just below them. 

 Below the third, and almost in front of the 

 medulla oblongata, is the fourth ventricle, 

 which connects with the canal in the spinal 

 cord. 



The Cerebrum-. The cerebrum occupies all 

 the upper part of the cranium, and forms over 

 seven-eighths of the brain. It is divided into 

 two equal parts called hemispheres by a deep 

 fissure extending from front to back. In shape 

 each hemisphere closely resembles a very large 

 coffee-bean. A band of fibers unites the hemi- 

 spheres on their under side and forms the roof 

 of the lateral ventricles. Each hemisphere is 

 divided into parts known as the frontal, the 

 parietal, the temporal and the occipital lobes, 

 each named from the bone of the skull under 

 which it lies. The brain substance in these 



