34 Drxon—On the Development of the Branches of the Fifth Cranial Nerve in Man. 
tissues surrounding it, deflecting or bending the primitive nasal nerve in Br;. 
The course of the frontal nerve in this and the next embryo bears out this 
explanation. In Rv. the frontal nerve has just appeared and is growing very 
rapidly. Its fibres growing out from cells of the Gasserian ganglion, after the 
more distal part of the nasal nerve has been bent forwards, run at first with the 
nasal fibres, forming with them a common trunk, but at the point where the nasal 
fibres bend forwards, the frontal fibres pass on as an independent nerve, and 
maintain the upward course of the common trunk for a short time. The upward 
course of the frontal fibres is, however, soon arrested; they also become bent in a 
forward direction, to form a nerve running above and parallel to the nasal, as we have 
observed in C.R. (figs. 7 and 8, Plate I.). The late appearance and rapid growth of 
the frontal fibres probably explains the direct course taken for some time by the 
frontal nerve. The first direction taken up by a nerve will be determined in part 
by the primitive direction assumed by the neuroblast fibres, and in part by the 
unequal growth of the tissues through which the nerve is passing; once, however, 
the fibres have been deflected, and have, for any reason, taken up a new course, 
these fibres will continue in this new direction, unless they are again deflected by 
unequal growth, or meet with a barrier through which they cannot pass. The 
turning inwards of the nasal nerve in C.R., near its termination, would seem to 
depend on the fact that while the distance between the nasal nerves of opposite 
sides, at their origins in C.R. is 0°5 m.m. greater than in Rv., yet the distance 
between the summits of the nasal pits is actually greater in Ru. than it is in C.R., 
owing to the approximation of nasal fossze towards the mesial plane, in the older 
stages. In this way, then, at the level of the nasal nerves, at all events, the head 
of the embryo, at this time, is increasing in width more posteriorly than in front. 
A separation of the proximal ends of the nasal nerves, without a corresponding 
separation of their more distal parts, will, of course, give the nerves a direction 
inwards. The nasal nerves finally turn downwards, their direction being 
determined by the growth downwards of the tissues to form the nose. 
The frontal nerves, placed at a higher level than the nasal, and passing close 
to the under surface of the cerebrum, do not become deflected inwards, but are 
on the other hand turned outwards, and follow the outward and upward growth of 
the hemisphere of the cerebrum. 
The infratrochlear branch of the nasal nerve comes off at the point where the 
nasal turns inwards, and continues at first, the direction of the main trunk. This 
would seem to show that the fibres which form this nerve grow out from the 
Gasserian ganglion at a later period, or, perhaps, at a slower rate than those of 
the nasal proper, and so escape the bending inwards experienced by the nasal fibres. 
In their growth the fibres of the infratrochlear leave the main nerve where it is 
sharply bent. It should be noted that in figure 8, Plate I., this point is not brought 
