The brain of Acipenser. 181 
small fasciculus with its accompanying cells is quite overtopped by 
the large acusticum, and the great number of arcuate fibres from 
the latter nucleus take the shortest course to the ventral raphe, so 
that most of them run mesial to the fasciculus communis. I regard 
this as only an example of nerve fibres being determined in their 
course by mechanical advantage. In Mammals the arcuate fibres pass 
on both sides of the fasciculus communis. 
At the caudal end of the medulla in the frog the communis 
bundles of the two sides approach one another as has been shown 
by STRONG in his fig. 31, and then decussate in a large commissura 
infima Halleri (Fig. W). This commissure is hence to be considered 
as a crossing of root fibres of the IX and X nerves to the other 
side. In my previous paper (98b) I stated that in Acipenser these 
are secondary fibres from the lobus vagi. This was probably an 
error: Among the decussating fibres are a considerable number of 
cells, which, in the only series in which they appear, have taken a 
peculiar rough impregnation. These cells form a definite unpaired 
nucleus, the nucleus of the commissure of CAJAL (see below). It has 
not before been described in lower Vertebrates. Continuing caud- 
ally from the commissure is an unbaired bundle of fibres (or pai- 
red bundles very closely apposed) lying upon the dorsal commissure 
and covered by the coarse medullated fibres of the dorsal tracts. 
No cells related to this bundle were impregnated and the fibres 
gradually grew less numerous and disappeared within one milli- 
meter. This corresponds to the fibres described above in Acipenser, 
Amia, Coregonus, and Catostomus and is probably homologous with 
CaJAL’s cervical bundle in Mammals. 
In Mammals, following the description of S. RAMÖN y CaJAL (96) 
which confirms the work of KÖLLIKER, HELD, CRAMER and earlier 
authors and adds some new facts, and is accepted by EDINGER, the 
fasciculus solitarius is a direct central continuation of the fibres of 
the sensory roots of the glossopharyngeus and vagus. ‘The fibres 
do not show true bifurcations but give off collaterals and end- 
branches to the two nuclei known as the uppper or outer sens- 
ory nucleus (ala cinerea) and the vertical nucleus (grey mass 
accompanying the fasciculus). The fasciculus solitarius divides into 
two bundles at the caudal end of the fourth ventricle: a) three- 
fourths of the fibres cross the middle line in the commissura infima 
Halleri and end in the nucleus of the commissure (CAJAL); b) the 
remainder of the fibres continue into the cervical cord giving off 
