34 
GEORGE E. NICHOLES. 
■while by otliei’S (Burckhardt, ’95, Studnicka, ’05) it is con- 
sidered as part of the dience-phalon, and as, so far as I can 
find, Houser nowhere even nieutions the posterior commissure, 
I am totally unable to decide -what exactly that author regards 
as the “anterior limit of the mid-brain,” but it seems probable 
that in this particular his account "was intended to agree 
closely with the account given by Sargent of the origin of the 
fibre in Eaia. The latter author had represented the fibre as 
emerging -svliolly behind the posterior commissure. 
Of the cells of the “ Dachkern,” whose axons were snpposed 
to unite to form Reissner’s fibre, Sargent had said (’01, p. 447) ; 
“ The cells are multipolar, giving off several processes in 
addition to the large axon, which is 2 p to 3 /u in diameter,” 
and “ The axons pass dorsad and laterad from their cells, 
turning either cephalad or caudad ” (my spaced type). 
Houser, more explicit, stated (op. cit., p. 129) that the axons 
from those of the cells in the anterior region of the tectum 
opticum pass cephalad to form Reissuer’s fibre, while those 
from the more posterior cells run caudad into the cerebellum. 
Neither he nor Sargent explained how such a very large 
number of these axons (in young Raia, according to Sargent, 
there are three or four hundred “Dachkern” cells with axons 
from two to three micra in diameter) could possibly become 
compressed into Reissner’s fibre, which has, according to the 
later account of Sargent (’04, p. 147), even in the adult only 
a diameter of G‘7 micra. Sargent, however, at a subsequent 
period, discriminated between finer axons running cephalad 
to constitute Reissner’s fibre and coarse neurites passing 
posteriorly to the cerebellum, and, ignoring his own earlier 
statements, dissented from Houser’s account, remarking (’04, 
p. 177): “I believe that each cell sends an axon anteriorly,, 
and also a cerebellar neurite posteriorly.” 
It is clear, moreover, from figures in Sargent’s later w'ork 
(’04) that he admits the existence in Raia, Mustelus and other 
Selachians of a relatively considerable part of Reissner’s fibre 
(the anterior branch) unrelated to the “ DachUern.” This runs 
forwards well beyond the point where the alleged fibre-tracts. 
