426 



HISTOLOGY 



motor cells on the same side, and thus is formed a motor path composed 

 of three neurones. Other tracts to the cord proceed from the cerebellum. 

 The motor nerves of the medulla oblongata, pons, and mid-brain arise 

 from groups of cells, or nuclei, which are typically near the median line and 

 only a short distance below the floor of the ventricle or cavity. Fig. 438 

 includes the nucleus of the hypoglossal nerve, which is in this position. 

 The lateral motor roots are further below the ventricle and are more lat- 

 eral. The nucleus ambiguus, which is an elongated structure containing 

 the motor cells of the accessory, vagus and glossopharyngeal nerves, is of 



this sort (Fig. 438). These 

 motor nuclei correspond with 

 cell groups in the ventral 

 columns of the cord, and 

 they are similarly in connec- 

 tion with fibers from the 

 pyramidal cells of the hemis- 

 pheres. In so far as the 

 latter pass to these cerebral 

 nerves, they form the cortico- 

 bulbar tract, "bulb" being a 

 general term for the ex- 

 panded part of the hind- 

 brain. The cortico-bulbar 

 fibers decussate at different 

 levels. 



Somewhat higher in the 

 medulla than the decussation 



of the descending motor fibers or pyramids, the sensory fibers ascending in 

 the gracile and cuneate fasciculi terminate in relation with groups of cells 

 known as the gracile and cuneate nuclei respectively (Fig. 437). They ap- 

 pear as additional horns of gray substance. The neuraxons from the cells 

 in these nuclei pass ventrally and decussate beneath the central canal, as 

 shown in Fig. 437. The bundles to which they give rise are known as the 

 medial lemnisci or fillets. In their course through the upper part of the 

 medulla, they are vertically placed bands of longitudinal fibers, on either side 

 of the median line (Fig. 438). The fillets not only receive fibers of muscle 

 sense through the gracile and cuneate fasciculi, but they are joined by the 

 spino-thalamic fasciculi of fibers of cutaneous sense, which pass up the 

 cord in the lateral funiculi. Moreover, they receive accessions from the 

 cerebral sensory nerves. The fibers of the latter enter the medulla and 

 divide into ascending and descending branches, like the dorsal root fibers 

 of the spinal nerves, but the descending fibers are relatively longer. The 

 position of the descending fibers of the trigeminal nerve (tractus spinalis 



lem. 



FIG. 438. SECTION OF THE MEDULLA. (After Dejerine.) 



c. r., Corpus restiforme; f. c. o., cerebello-olivary fibers; lem., 

 lemniscus or fillet; n. am., nucleus ambiguus; n. h., nu- 

 cleus hypoglossi; ol., olive; py., pyramid; t. s., tractus 

 solitarius; t. s. n. t., tractus spinalis nervi trigemini; v., 

 fourth ventricle. 



