THE NERVOUS SYSTEM 299 



body. Acts of thinking, too, seem to be somewhat weakened, 

 becoming less virile and positive, in the cases where the 

 cerebellum has been impaired by disease or some other cause. 

 However, this function of the cerebellum as a message- 

 strengthening organ cannot be regarded as so important, or 

 so certainly known, as its co-ordinating influence. 



THE MEDULLA 



The medulla is connected with the control of respiratory 

 and circulatory organs. It lies beneath the cerebellum and 

 its lower end is continuous with the spinal cord from which 

 it is not distinctly separate. Large bundles of fibres, crura 

 cerebri, extend from its upper end into the cerebrum. 



If the tissues of the medulla were carefully examined, it 

 would show a complex mixture of nerve fibres and nerve cells, 

 whose arrangement would differ very much, depending on 

 whether the cut were made through the anterior, middle or 

 posterior portion of the structure. We need merely note 

 that among these fibres there are patches of cells, sometimes 

 called " nuclei," from which most of the cranial nerves take 

 their origin. The fibres themselves come from cells which 

 are either in the cerebrum or lower down, in the cord; so that 

 the medulla becomes a great complex of paths for messages 

 passing in either direction. 



So far as its fibres are concerned, the medulla simply trans- 

 mits messages from the brain down to the spinal cord, and in 

 the reverse direction, but its nerve cells give it some other 

 functions. The particular activities which are controlled 

 by the nerve cells of the medulla have been determined by 

 removing from some animal the other parts of the brain and 

 then noting carefully what powers have been taken away and 

 what powers are left. Such an animal keeps on breathing, and 

 the blood vessels still continue to expand and contract, so we 

 say that the medulla contains respiratory and vaso-motor 

 centers. In the medulla is also the cardio-inhibitory center, 



