FUNCTIONS OP RELATION NERVOUS SYSTEM. 47 



on cutting open the cerebellum of a sheep, and was by 

 the early anatomists called the tree of life. The cere- 

 bellum has no interior cavity, but below it and in front 

 are two ventricles that communicate with the ventricles 

 of the cerebrum. 



The medulla oblongata connects both the cerebrum 

 and the cerebellum with the spinal cord. From the pos- 

 terior base of the cerebrum and from the cerebellum 

 project large columns that appear like stems, and are 

 called cerebral peduncles. Below is a thick transverse 

 band or bridge, called pons Varolii, and above, between 

 the cerebrum and cerebellum, are little eminences, called 

 optic thalami or quadrig-eminal tubercles. The union of 

 the peduncles of the cerebrum and those of the cerebel- 

 lum forms the rachidian bulb. The fibres of this bulb, 

 six in number, four in front and two behind, are direct 

 continuations of those of the spinal cord. The middle 

 two in front, or anterior pyramids, as they are called, 

 come from the cerebrum, and are crossed in such a 

 manner that those which arise in the right hemi- 

 sphere form the left pyramid, while those having 

 origin in the left hemisphere constitute the right- 

 pyramid. 



This crossing explains why paralysis that affects one 

 hemisphere of the brain produces loss of sensibility or 

 of movement on the opposite side. 



The cranial nerves, so called because they come directly 

 from the brain, inside of the skull, form the first twelve 

 pairs of the forty -three pairs in the cerebro-spinal sys- 

 tem. Their functions are very different, comprising the 

 special senses, and the mobility and sensation of different 

 parts of the face. One of them, the pneumogastric, 

 passes down into the thorax and abdomen, and plays a 



