78 THE NERVOUS SYSTEM. 



of the anterior cerebrals by a posterior communicating 

 artery. The two anterior cerebrals also are joined by 

 an anterior communicating artery, thus completing the 

 circle. The circle thus formed at the base of the brain 

 is called the circle of Willis and provides for a good 

 supply of blood in event of an accident to any vessel. 

 The blood is returned to the general circulation through 

 the cerebral veins and sinuses formed by the separation 

 of the dura mater into two layers. 



The cerebellum is about one-seventh the size of the 

 cerebrum and weighs about 5 ounces. It lies in the 

 lower occipital fossae of the skull and is oblong in shape 

 and divided into two lateral hemispheres by a trans- 

 verse fissure. It is made up of both white and gray mat- 

 ter, of which the former predominates, the gray being 

 external as in the cerebrum. The cells are about the 

 same as in the cortex and its surface is traversed by 

 queer furrows. Of its function little is known but it 

 probably plays a most important part in the coordi- 

 nation of the nervous and muscular acts by which the 

 movements of the body are carried on. 



At the back of the cerebrum and below the cerebel- 

 lum is the pons Varolii, which forms a connecting link 

 with the medulla oblongata or bulging part of the cord. 

 It is made up essentially of white matter or nerve 

 fibers, though there is a small amount of gray matter 

 in which are found the nuclei of some of the cranial 

 nerves. 



In the medulla oblongata, which is about 1 inch long 

 and extends from the pons Varolii to the upper border 

 of the atlas or first cervical vertebra, the gray matter 

 is not necessarily external to the white but is found in 

 patches in the white. The gray matter here corresponds 

 more or less to that of the spinal cord and the white 

 matter is continuous with that of the cord. From the 

 medulla arise the fifth to twelfth cranial nerves and the 

 vasomotor nerves. The cardiac nerve has its center here 

 and here too are the centers of respiration, phonation, 



