714 THE NERVOUS SYSTEM. 



is thus separated from the base, and becomes pushed over laterally. At first it is 



Nucl. gracilis 

 Nucl. cuneat., 



Cornu post.~H 

 Decussation^^. 



Cornu ant. ^f 



Pyramid 



FIG. 418. Transverse section through the begin- 

 ning of the medulla X \ . (Gegenbaur.) 



Nucl. gracilis 

 Nucl. cuneat.^ 



Cornu post. 



FIG. 419. Transverse section through the closed 

 portion of the medulla, at a higher level than the pre- 

 ceding X f . (Gegenbaur.) 



Pyramid 



Cornu post. 

 Eaphe 



Main oli- 

 vary nucleus 



XII. 



Accessory olivary nucleus 



FIG. 420. Transverse section of the 

 medulla at the junction of the open and 

 closed portions X f. (Gegenbaur.) 



somewhat distinct, but as seen in sections immediately above (Fig. 420) it rap- 

 idly becomes disintegrated, as it were, into the gray 

 gracilis matter of the formatio reticularis of the anterior 



and lateral areas (see above). The base of the 

 cornu remains as a portion of gray matter close to 

 the ventro-lateral aspect of the central canal. The 

 lateral horn (Fig. 417) of the cord is also somewhat 

 isolated, and is seen in the lateral area near the 

 surface as the nucleus lateralis. 



The posterior cornu (Figs. 418, 419, 420) is 

 changed thus : The caput of the posterior horn 

 becomes enlarged, and gradually shifted outward, 

 so that it forms a rounded mass, which produces 

 the prominence on the surface called the funiculus 

 of Rolando and its tubercle. The neck of the cornu 

 diminishes in size, and is broken up into a reticular formation, which blends with 

 that derived from the anterior cornu, by the passage of longitudinal and trans- 

 verse fibres through it, so that the caput is separated from the rest of the gray 

 matter. 



Just before and as the central canal expands into the fourth ventricle the base 

 of the posterior horn of gray matter is pushed outward into the funiculus cuneatus 

 and funiculus gracilis ; in each of these funiculi it forms a distinct accumulation 

 of gray matter, constituting the nucleus cuneatus and the nucleus gracilis. These 

 nuclei may be regarded as helping to form the "formatio reticularis " of the pos- 

 terior area, although the reticular appearance is much less marked than in the 

 lateral or anterior area. On the surface these nuclei produce, respectively, the 

 cuneate tubercle and clava. A small portion of the base of the posterior horn is 

 separated from the remainder, and is placed lateral to the cuneate nucleus ; it is 

 known as the accessory cuneate nucleus, probably derived from Clarke's vesicular 

 column (gray matter) of the cord. Fibres from this nucleus run to the restiform 

 body. 



The white matter of the closed portion of the medulla is made up of white 

 fibres, some collected into large bundles on the surface, while others are found in 

 the formatio reticularis. The latter, being directly continued upward into the 

 fibres of the formatio reticularis of the upper or open portion of the medulla, 

 will be taken up in the description of that region. 

 The fibres on the surface : 



Of these the decussation of the pyramids, the "lateral tract," and direct 

 cerebellar tract have been already dwelt upon. They will again be referred to, 

 however, in connection with the upper part of the bulb. 



The funiculus of Rolando is due to the enlarged head of the posterior cornu of 

 the gray matter, which is displaced laterally in consequence of the increase in 

 size of the posterior columns of the medulla, so that it lies almost at right angles 



