THE NERVOUS SYSTEM 



1353 



called the formatio reticularis alba. It contains little grey matter 

 and few nerve-cells. 



Central Grey Matter. — ^The grey matter which surrounds the 

 central canal in the lower or closed part of the bulb is derived from 

 the basal portions of the ventral and dorsal grey comuaof the upper 

 part of the spinal cord. In the upper or open part of the bulb this 

 central grey matter spreads out and forms a thick layer over the 

 lower or bulbar part of the floor of the fourth ventricle. The 

 mesial part of this layer represents the basal part of the ventral giey 

 comu, and it contains the hypoglossal nucleus. The lateral part 

 represents the basal part of the dorsal grey cornu, and it contains 

 pneumogastric, glosso-pharyngeal, and auditory nuclei. 



Fig. 566. — Section of the Medulla Oblongata at the I>evel of the 

 Decussation of the Pyramids (L. Testut's * Anatomie Humaine '). 



1. Anterior Median Fissure 



2. Posterior Median Fissure 



3. Motor Roots 



4. Sensory Roots 



5. Base of the Anterior Cornu from which the 



Caput, 5', has been detached by the pas- 

 sage of the crossed Pyramidal Tract 



6. Decussation of the crossed Pyramidal 



Tracts 



7. Posterior Cornu 



8. Nucleus Gracilis 



Substantia Gelatinosa of Rolando.— This caps the detached and 

 displaced caput of the dorsal horn of grey mattei . Having increased 

 in amount and lying close to the surface, it gives rise to the funiculus 

 and tubercle of Rolando. 



Nuclei of Grey Matter.— The nuclei, which will be considered in 

 this place, are as follows : 



1. Nucleus gracilis, 



2. Nucleus cuneatus. 



3. Ohvary nuclei. 



4. Arcuate nucleus. 



5. Nucleus lateralis. 



The nucleus gracilis is a collection of grey matter within the 

 funiculus gracilis. For the most part it is connected with the grey 



