THE MESENCEPHALON AND CEREBELLUM. 637 



made up exclusively of conducting fibres. The gray substance 

 may be said to be distributed in four more or less distinct regions. 

 (1.) Of these one can be traced along the floor of the fourth ven- 

 tricle, from the gray matter of the cord to the base of the brain, 

 as far forwards as the tuber cinereum, so that it may be said to 

 be representative of the gray matter forming the inner lining of 

 the primitive nervous tube. (2.) Then come the ganglia of the 

 brain, which are the more or less isolated masses of gray sub- 

 stance within the brain, known as the corpora quadrigemina, 

 optic thalami, corpora striata, etc. (3.) The gray substance of 

 the cerebellum and of the corpora quadrigemina is derived from 

 the upper part of the mid-brain. (4.) The cortex of the hemi- 

 spheres of the brain forms the most extensive gray district, and 

 must be regarded as quite distinct from the preceding. 



Connecting the various parts of these gray regions are sets of 

 fibres, which may be classified as follows : 



1. Those which act as channels of intercommunication between 

 the different parts of the same region. These may be divided 

 into unilateral, which connect together the cells of a single hemi- 

 sphere, and bilateral, or commissural fibres, which unite the cor- 

 responding masses of gray matter on the two sides of the brain. 



2. Those which connect the different regions one with another. 

 Under this head naturally fall (1) those fibres which pass between 

 the cortex and the basal ganglia ; (2) those running from the 

 cortex to the cerebellum ; and (3) those connecting the above 

 with the axial or spinal gray matter. 



THE MESENCEPHALON AND CEREBELLUM. 



In examining the functions of the brain, it will be advantageous 

 to consider the various parts in the order they are found in pro- 

 ceeding from the medulla towards the cerebral hemispheres. 

 Between the medulla oblongata and the hemispheres, we thus 

 come to a group of parts, including the pons, the corpora quad- 

 rigemina, pons varolii, and cerebellum, which may be called the 

 mesencephalon, being developed from the mid-brain. The duties 

 of this part of the nervous centres can be investigated by observ- 

 ing the actions of lower animals in which the hemispheres have 



