THE NERVOUS SYSTEM 1351 



lower or closed part of the bulb. Inferiorly the funiculus of 

 Rolando is narrow, but it w'idens as it ascends, and superiorly it 

 terminates in an enlarged extremity, called the tubercle of Rolando. 



The funiculus and tubercle of Rolando are covered by a thin layer 

 of longitudinal ner\-e-fibres which represent the spinal or descending 

 sensory root of the fifth cranial ner\'e. 



Upper Portion of Posterior Area. — This portion, as stated, per- 

 tains to the upper or open part of the bulb, and extends as far as 

 the lateral boundary of the lower or bulbar half of the floor of the 

 fourth ventricle. It presents a prominent round tract, called the 

 restiform body {corpus restiforme B.), which is situated between the 

 lower half of the floor of the fourth ventricle and the funiculi of the 

 pneumogastric and glosso-phar^mgeal nerves. Its direction is 

 upwards, outwards, and backwards, and it enters the corresponding 

 hemisphere of the cerebellum. It is otherwise known as the inferior 

 cerebellar peduncle. 



The restiform body succeeds to the funiculus gracilis and funiculus 

 cuneatus of the low-er portion of the posterior area of the bulb, but 

 it is quite distinct from these funiculi, and receives no fibres from 

 them. The sources of its fibres wall be stated in connection with 

 the internal structure of the bulb. Meanwhile it may be stated 

 that it constitutes the great tract of connection between the cere- 

 bellar hemisphere, the bulb, and the spinal cord. 



The restiform body becomes conspicuous above the level of the 

 cuneate tubercle, and forms the lateral boundary of the lower or 

 bulbar half of the floor of the fourth ventricle. 



Internal Structure of the Medulla Oblongata. — Each half of the 

 bulb is composed of grey nerv^ous matter and tracts of white nerv'ous 

 matter. 



Grey Matter. — The grey matter lies largely in the interior. Over 

 the dorsal aspect of the upper or open part of the bulb, however, 

 it comes to the surface and covers the lower or bulbar half of the 

 floor of the fourth ventricle. 



As compared with the grey matter of the spinal cord, it presents 

 important modifications, and its component parts Eire as follows: 



1. Substantia or formatio reticularis. 



2. A thick layer of grey matter around the central canal in the 



lower or closed part of the bulb. 



3. A thick layer of grey matter over the floor of the fourth 



ventricle in the upper or open part of the bulb. 



4. Substantia gelatinosa of Rolando. 



5. Nuclei of grey matter. 



The modifications undergone by the grey matter of the bulb in 

 its lower or closed part are brought about by the decussation of 

 the pjTamids. The ner\'e-funiculi of the crossed pyramidal tract 

 of the spinal cord, on one side as they are traced upwards, pass 

 through the ventral grey cornu of that side, and then cross in the 

 lower part of the ventral median fissure of the bulb to the pyramid 



