116 NERVOUS SYSTEM. 



on its way to distribution in the cerebellum ; while the third set of 

 fibres goes to the fasciculus teres, and reaches the ganglia at the 

 base of the brain. (See Fig. 32.) 



What is the mode of connection of the posterior column of the 

 cord? 



Its fibres continue on as the posterior pyramid of the medulla 

 and restiform body. The former probably communicates with the 

 basal ganglia, though it has not been traced as far, and the latter 

 (restiform body) for the most part reaches the cerebellum.* 



What are the olivary bodies ? 



Each is a mass of white nerve-substance containing a central 

 gray nucleus. There are communications between it and some of 

 the tracts from the cord, especially from those tracts of the ante- 

 rior and lateral columns which go to the ganglia at the base of the 

 brain. 



How is the gray substance arranged in the medulla oblongata ? 



As the fibres which form the crossed pyramidal tract pass from 

 the lateral tract to decussate into the anterior pyramid of the 

 opposite side, they push the anterior cornu of the gray matter 

 backward ; and this is still further accomplished by the olivary 

 body, until the gray matter is spread out upon the posterior sur- 

 face of the medulla at its upper part. Here the central canal of 

 the cord has widened out to form the fourth ventricle, and the 

 gray substance is aggregated to form the floor of the ventricle. 

 There are some other collections of gray matter for example, in 

 the olivary bodies but this is the principal accumulation. 



What especial importance has this collection of gray substance in 



the floor of the fourth ventricle ? 



In this nucleus are the origins of some of the cranial nerves : 

 the spinal accessory, hypoglossal, pneumogastric, andglosso-pharyn- 

 geal nerves, and a root of the auditory of the facial, and of the 

 trigeminus nerves, arise in this important collection of gray matter. 



* In speaking of these fibres it has been convenient to say that they 

 "pass "in certain directions or "are distributed" in some situation. It 

 must not be forgotten that they are afferent and efferent medullated nerve- 

 fibres, and that such terms must be considered as somewhat figurative. In 

 reality, it would not seem proper to speak of an efferent fibre as being " dis- 

 tributed " at its origin, but convenience and usage permit the use of these 

 and similar expressions. 



