MAMMALIAN. Qi 



CHAPTER IV. 



THE ABDUCENS NUCLEUS AND EOOTS. 



The course and origin of the roots of the sixth pair of nerves have been described 

 by Stilling^ and Schroder van der Kolk," but their accounts differ very considerably. 

 The general course of these nerve-roots before they reach the nucleus of the facial 

 has been given with sufficient detail in a previous chapter, where it was also shown 

 that they form a remarkable exception in their inward course, to all the other 

 nerve-roots, bending outivarcl as they approach the ffoor of the fourth ventricle, 

 while aU the other nerves of the medulla bend inioards or towards the raphe, Avhere 

 their fibres decussate to a greater or less extent with those coming from the opposite 

 side. As a natural consequence resulting from their peculiar course, the fibres 

 constituting the abducentes roots do not decussate, at least directly, and if the 

 opposite nuclei are brought into connection it must be either through the interven- 

 tion of fibres derived from cells, or by means of ascending or descendiiig fibres 

 connecting the nucleus with distant parts, for I have not been able to trace any 

 fibres turning inwards towards the raphe. 



According to Stilling the roots of the sixth nerve end in the same nucleus to which 

 we have traced the roots of the facial, making this large group of cells a common 

 nucleus for the two motor nerves of the trapezium. Schroder van der Kolk, on the 

 other hand, does not admit that the abducens takes its origin from the same nucleus 

 as the facial, and remarks that it would be very singular if two nerves so distinct 

 in their action should arise from a common nucleus. This observation Avould seem 

 to have but little weight, since we constantly find in the medulla, nerves very dis- 

 tinct in their action arising from the same column, the nucleus of the one changing 

 so imperceptibly into the nucleus of the other, that it is entirely impossible to state 

 with any approach to accuracy Avhen or how the change takes place. Schroder van 

 der Kolk states that the roots of the sixth penetrate those of the facial, after pass- 

 ing through the nucleus, and terminate on the floor of the fourth ventricle. Their 

 further course he seems to have been unable to trace, but surmises that they may 

 probably turn upwards, entering into close connection with the nucleus of the oculo- 

 motor nerve which is situated above. 



According to my own observations, the roots of the sixth pair of nerves pass 

 directly inward, in two or three somewhat curving bundles, which pursue a slightly 

 ascending course until they nearly reach the floor of the fourth ventricle (Plates III 

 and IV, Figs. 11 and 15, Plate XVI, Figs. 12" and 44). As they pass along the 



> Pons Yarolii, 36 and 153. ' Medulla Oblongata, 120 et seq. 



