300 
MR. J, L. CLARKE ON THE INTIMATE STRUCTURE OF THE BRAIN. 
on reaching the commencement of the loop, spread out, and bend forward and inward, 
but, at the same time, obliquely downward, so as to enter different parts of the nucleus 
by a series of drooping bundles along the under surface of the loop, as seen at the end 
of the line (Q'"). A transverse section along this part of the nerve is represented in 
tig. 51, Plate XIII. If we turn this figure on its right side with the plane of the paper 
perpendicular to the plane of that on which fig. 55 is represented, we shall have a good 
idea of the course of these fibres, provided we recollect at the same time that they run 
somewhat obliquely downward or to the right. In consequence of this downward course, 
the fibres of the nerve, as they enter the nucleus Q ", fig. 51, Plate XIII., are observed, 
in transverse sections, to be shaved off obliquely at different lengths. Moreover, it will 
be seen that all these fibres belonging to the lower side of the nerve-trunk enter different 
parts of the nucleus from without inward, the innermost meeting and intimately mixing 
with those of the abducens nerve (V'), as they bend forward on the outer side of the oval 
column or loop (T"). It is evident, therefore, that none of them contribute to form the 
longitudinal column (T"), although a few pass in front of it to decussate across the raphe 
with their fellows of the opposite side. 
(54) On examining a similar section of the medulla a little higher up, as at v 1 ", for 
instance, fig. 55, Plate XIII., we find that the nerve divides into two portions, a super- 
ficial and deep portion, as already stated, and as represented at 1! g", fig. 56. The fibres 
belonging to its deep portion (q") bend forward and inward, as in the former case, and 
enter the nucleus (Q"), which is now somewhat diminished, and where they meet the 
network of bundles proceeding from the abducens nerve (V'). The fibres of the other 
portion, on reaching the fasciculus teres , curve downward without entering the nucleus, 
and thus contribute to form the longitudinal part or summit of the loop, the trans- 
versely-cut end of which is shown at T". In the transverse section, therefore (fig. 56), 
they are seen to enter the cut-end of this longitudinal part (T"). Before they enter it, 
however, their arrangement, as already stated, is very curious, and doubtless very impor- 
tant. Many of the fibres from the deep or anterior layer (q") of the nerve are seen to 
run obliquely backward and inward to the posterior or upper layer (V) of the longitudinal 
column (T"), where they bend round and run downward ; while the fibres forming the 
superficial or posterior layer (?) of the nerve run obliquely forward and inward to enter the 
middle and anterior layers (T" V') of the longitudinal column, and run downward with it, 
like the others. Some of them, however, continue their course transversely in front of 
the column to decussate with their fellows of the opposite side, across the median raphe 
(at h). It is evident, therefore, that just before these two sets of fibres bend round and 
contribute to form the longitudinal portion of the loop, they decussate each other (as 
shown in fig. 56, W" W'") immediately behind the facial nucleus (Q"), with which the 
fibres forming the deep or anterior layer of the nerve are connected*. This, however, 
* This decussation is crossed nearly at right angles by roots of the abducens nerve proceeding to the super- 
ficial grey layer (g", g" 1 ), and by other fibres which proceed from this layer to the nucleus (Q"), and which are 
particularly numerous in the Rabbit, Sheep, and other animals. 
