76 J. B. JOHNSTON, 
parallel with the spinal V tract and mesial to it. This bundle will 
be referred to below as the spinal VIII (page 80). 
The spinal V tract (Phots. 8—14) forms a compact bundle at 
the very surface of the medulla and immediately ventral to the 
motor roots of the IX and X nerves. As it runs forward small 
bundles leave this tract to go out with several of the dorsal roots 
of the X nerve, and a large bundle goes out with the dorsal root 
of the IX nerve. In the region of the IX and X nerves the spinal 
V tract is separated from the acusticum by both dorsal and ventral 
roots of those nerves, while cephalad from the IX it comes in con- 
tact with the bundles of the VIII and lateral line fibres in the 
acusticum. It is still easily traced, however, as a very compact 
bundle, forward to the root of the trigeminus (Trig. I dors. of 
GORONOWITSCH), with which the entire bundle passes out of the 
brain (Phot. 14). The spinal V tract is, therefore, a tract of de- 
scending fibres, which take their origin in greater part from the cells 
of the Gasserian ganglion and in lesser part from some of the 
cells of the vagal and glossopharyngeal ‘ganglia, enter the medulla 
ana take a course caudad to end in a nucleus common to them and 
the dorsal funiculi of the cord. The fibres of the descending tract 
are seldom impregnated in my preparations except as they approach 
their nucleus of ending. For this reason it is difficult to determine 
whether any fibres leave the tract to find other endings. Many 
medullated fibres of fine calibre which seem to come from this 
bundle course around the lateral surface of the medulla, cross the 
median raphe and presumably enter the lateral columns of the other 
side. Arcuate fibres from the acusticum, as will be seen later, take 
the same course in some cases, and it is probable that all these 
fibres are of the latter character. Fine fibres running from among 
the spinal V fibres to the lower olive have been mentioned in de- 
scribing that nucleus. 
The spinal V tract represents only the smaller part of the root 
of the trigeminus from which it is derived. The remainder plunges 
deeper into the medulla and divides into ascending and descending 
portions. The transverse section of each of these portions is much 
greater than that of the superficial descending tract, but they are 
not so compact. The deep descending tract runs parallel with the 
superficial one, but near the central cavity. It is diffuse and mingles 
somewhat with the bundles of VIII and lateral line fibres by which 
it is separated from the superficial tract. In my preliminary com- 
