586 MANUAL OF HISTOLOGY. 



terior roots of the spinal nerves, the medulla oblongata was found by him 

 to possess an additional one, namety, from a third lateral tract. This 

 commences so low down as the upper -part of the spinal cord (with the 

 farther development of the so-called lateral accessory cornu) as a narrow, 

 separate, nervous bundle. 



To these three systems of roots all the nerves of the medulla oblongata 

 may be referred. 



(a.) From the lateral system several nerves spring. The first of these is 

 the accessorius, which is soon followed by the vagus and glossopharyngeus. 

 This place of origin of the lateral system is originally nothing but a 

 special division of the anterior cornu for the accessory nerve. To this 

 there is soon added a portion of the posterior sensitive cornu (which may 

 be followed up as far as the pons), so that the nerves springing from this 

 lateral part may be of mixed nature. 



The facialis and acusticus also, as well as the anterior root of the 

 trigeminus, take their origin likewise from this lateral tract of the 

 cineritious substance. This surprising relation, however, is explained 

 by the fact, that each of them again splits up into a sensitive portion 

 (acusticus), and a motor (anterior root of trigeminus and facialis). 



(b.) The sensitive portion of the trigeminus, on the other hand, is 

 derived from the posterior root system. The fibres of the latter are like- 

 wise collected, from the first spinal nerves on, into longitudinal tracts, 

 which do not, however, leave the medulla oblongata as separate sensitive 

 bundles, as in the cord, but unite to form this root. 



(c.) Besides the hypoglossus, the abducens, trochlearis, and oculomo- 

 torius, all correspond to the anterior spinal roots. 



As to the nuclei, already mentioned, of the numerous nerves arising 

 here, those of the hypoglossus and accessory first appear most inferiorly, 

 appertaining to the deepest portion of the anterior cornu, and situated in 

 the neighbourhood of the central canal. These are collections of multi- 

 polar motor ganglion cells in the cineritious substance, which, as has been 

 already mentioned, is split up here and reticulated. Then there make 

 their appearance successively on the floor of the fourth ventricle and 

 around the aqueduct of Silvius, similar spots for the vagus, glossopharyn- 

 geus, abducens, trochlearis, and oculomotorius. 



Let us turn again, for a few moments, to the nucleus for the hypo- 

 glossus. Its large multipolar ganglion cells present, like those of the 

 anterior cornu of the grey matter of the spinal cord, protoplasm ramifications 

 and an axis cylinder process, which probably becomes eventually a fibre 

 of the hypoglossus (Gerlach). It was at one time generally supposed that 

 a complete decussation of the hypoglossal fibres took place here. This 

 is, however, more probably only partial (Clarke, Dean, Deiters). The 

 arrangement, as found by Gerlach, is as follows : In the first place, a set 

 of delicate fibres, lying posteriorly, passes across from one hypoglossal 

 nucleus to the other, and may be regarded as commissural between the 

 nuclei themselves. Other fasiculi, again, lying more anteriorly at the 

 bottom of the raphe, and of greater diameter, decussate, as proper roots of 

 the hypoglossal nerves, with the fibres of the opposite side, pursuing their 

 course in the trunk of the side opposite to that on which they took origin. 



The external portion of the posterior cornu likewise remains almost 

 unchanged in its grey substance, and the connection between it and the 

 motor nucleus of the middle line is still continuous and diffuse. It gives 

 origin to the sensitive trigeminvs root and the accusticus, which does not, 



