ROOTS OF THE VAGUS GROUP. 95 



sensory root of the IX, which passes from the fasciculus 

 communis to its exit dorsally of the spinal V tract. 



The entire course of the motor IX fibres can be followed 

 with precision from the nucleus of origin to the peripheral 

 distribution. The general arrangement is essentially as 

 described by Mayser in cyprinoids ('8i). In the carp it 

 does not form so sharp a genu and passes out dorsally of 

 most of the fibres of the spinal V tract. Johnston ('98) 

 describes the motor IX as arising wholly from the fas- 

 ciculus longitudinalis dorsalis, evidently not having traced 

 the fibres back to their nucleus of origin. 



iv. — General Considerations. — Two classes of motor 

 fibres emerge with the vagus of fishes, (i) coarse fibres 

 from the nucleus ambiguus for the striated branchial mus- 

 culature, (2) very fine fibres for the unstriated visceral 

 musculature, such as the constrictor muscles of the lower 

 part of the oesophagus, etc. 



The nucleus ambiguus in these cases is the undoubted 

 homologue of the nucleus of the same name in the mam- 

 mals. The chief nucleus of the vagus in the mammals is 

 commonly regarded as sensory (Van Gehuchten, '97, p. 

 481; KoUiker, '96, p. 239), yet Forel ('91) brought forth 

 very strong evidence by v, Gudden's method tending to 

 show that it is motor, while the descending, or spinal 

 nucleus, which accompanies the solitary bundle, is sen- 

 sory. In this he may be too extreme; nevertheless the 

 probability remains very strong that in the mammals the 

 chief vagus nucleus contains some motor cells. (Compare 

 the more recent work of Van Gehuchten cited in Section 



3, ni). 



We get a hint along the same line from the degenera- 

 tion experiments of Niedzvietzky ('97, p. 534). After re- 

 section of the vagus in three adult rabbits he found cells 



