956 NUCLEI OF CRANIAL NERVES. [Boon m. 



part, fibres sweep in a ventral and lateral direction through the 

 dorsal part of the reticular formation, pass ventral to, or in some 

 cases through the gelatinous substance and the. strand of fibres 

 connected with the fifth nerve (Fig. 109, v. a), and reach the surface 

 of the bulb on its lateral aspect in a line between the olivary and 

 restiform bodies (Fig. 108, c). Along this line may be seen (Fig. 



108, c.) a series of roots ; of these the lowest, the accessory roots, 

 spring from the hind part, the highest, the glossopharyngeal roots, 

 from the front part (and it is these especially which pierce the 

 gelatinous substance (Fig. 110, IX. a)), and the intermediate, the 

 vagus roots, from the middle part of the combined nucleus. Hence 

 we may speak of the hind part of the whole nucleus as being the 

 accessory nucleus, the middle part as the vagus nucleus, and the 

 front part as the glossopharyngeal nucleus. 



All the fibres however of the roots of these three nerves do not 

 take origin from the nucleus in question ; some of the fibres start in 

 a different way. In sections of the bulb above the decussation of 

 the pyramid a patch of grey matter is seen lying in the lateral 

 part of the reticular formation (Fig. 109, x. m), about midway 

 between the ventral and dorsal surfaces. What is thus disclosed 

 by sections is a column of grey matter, the " nucleus ambiguus " 

 (Fig. 115, no), stretching about as far forwards and backwards as 

 the combined accessory-vago-glossopharyngeal nucleus, but placed 

 distinctly more ventrally and somewhat more laterally. (In Fig. 

 115, it and the combined nucleus are represented on different 

 sides of the diagram, to avoid confusion through the overlapping 

 of the shading.) From it fibres curve round (Fig. 109, 6, x. m), to 

 join the accessory- vago-glossopharyngeal roots, but especially the 

 vagus roots. It may therefore be considered as a second nucleus 

 of the vagus (and possibly of the other) roots. 



But there is yet a third source of some of the fibres of the 

 nerves of which we are speaking. In sections through the bulb 

 there may be seen just ventral to and a little lateral to the 

 combined nucleus (Fig. 109, 4, 5, 6, IX. a), the circular section 

 of a longitudinal bundle of fibres. In the hinder sections (Fig. 



109, 4) the bundle is a very thin one and still further back it 

 is lost to view, though there are reasons for thinking that some 

 of the fibres are continued back into the cervical cord, as far as 

 the origin of the fourth cervical nerve or even beyond ; in the 

 more forward sections (Fig. 109, 5 and 6), it increases in diameter 

 and may be traced forward to the front end of the combined 

 nucleus into which it merges. It is a bundle of fibres which, 

 starting successively in the lateral grey matter of the cervical cord 

 and higher up in the reticular formation of the bulb, run longi- 

 tudinally forwards; the bundle at first increases in size by the 

 addition of fresh fibres at each step ; but farther forwards the 

 fibres leave the bundle to pass into the roots of the nerves 

 of which we are speaking, especially of the glossopharyngeal, 



