THE NERVOUS SYSTEM 107 



gray matter imbedded in the cerebrum, the first inti- 

 mately connected with the anterior part of the internal 

 capsule and the latter with its posterior third. Since 

 these bundles carry respectively motor (efferent) and 

 sensory (afferent) impulses, the corpora are supposed 

 to be connected with motion and the thalami with sen- 

 sation. 



Four small bodies corpora quadrigemina are sit- 

 uated just back of the thalami. The first pair are con- 

 nected with vision and the posterior with hearing. 



The cerebellum is so much larger than the portions of 

 brain just discussed that mere size would indicate its 

 possession of important functions. We are, however, al- 

 most entirely ignorant of its work and so many theories 

 have been advanced that one hesitates to speak of 

 knowledge of the subject. Its removal certainly causes 

 a loss of the power of coordinating voluntary muscular 

 action in animals; so that if a coordinating center for 

 this purpose exists, it is highly probable that it is located 

 in the cerebellum, and aids in maintaining equilibrium. 



The Medulla Oblongata. Here we are on firmer 

 ground. This somewhat pear-shaped body seems to con- 

 tinue the conducting fibers from all parts of the brain 

 lying above it into the spinal cord. Its conducting func- 

 tion, a large part of its work, is, therefore, obvious. The 

 paths of conduction of motor impulses are in the an- 

 terior pyramids, whose fibers cross to the opposite side 

 of the cord as the crossed pyramidal tracts explaining 

 why an injury to the motor area of one Cerebral hemi- 

 sphere causes paralysis of the other side of the body. 



The sensory fibers finally pass through the medulla 

 into that region of the cord around the posterior roots 

 of the spinal nerves with which they are connected. 



