218 OUR BODIES AND HOW WE LIVE 



The active powers of the mind are supposed to reside in 

 this outer layer of the brain. These powers are great or 

 small, according to the number and the extent of its folds, 

 or convolutions. In the lowest vertebrate animals the brain 

 has no folds ; but as we pass to animals of a higher grade, 

 the folds begin to appear. 1 



295. The Cerebellum. The cerebellum, or little brain, lies 

 beneath the back part of the brain proper. It is made up 

 of two halves, each formed of a number of layers of gray 

 and white nerve matter, curiously arranged. These masses 

 resemble somewhat, in section, the branches of a tiny tree 

 and hence are fancifully called "arbor vitas " (Fig. 136). 



The functions of the cerebellum are not yet certainly 

 known. It appears to aid in the control of the muscles of 

 the body; that is, it serves to bring the various muscular 

 movements into harmonious action. 



296. The Medulla Oblongata. The medulla oblongata is 

 the thick continuation of the spinal cord lying within the 

 cavity of the skull. It is just under the little brain, and 

 makes the connecting link between the brain and the spinal 

 cord. It is a highly important part of the brain, since from 

 it arise important nerves which regulate breathing, swallow- 

 ing, the heart's action, and other vital processes. If this 

 part of the brain be broken or cut, respiration and circula- 

 tion will at once cease, causing instant death. 



1 The brain is inclosed within three distinct membranes, the dura 

 mater (hard mother), the arachnoid (like a spider's web), and the pia 

 mater (delicate mother). 



The dura mater is the tough membrane which lines the inner surface of 

 the skull and forms a loose outer covering for the brain. The middle 

 layer, called the arachnoid, secretes a fluid which keeps the inner surface 

 moist. The pia mater is a very delicate membrane which dips down between 

 the folds of the cerebrum. 



