Dixon— On the Development of the Branches of the Fifth Cranial Nerve in Man. 33 
The above diagrams (fig. 1) show that in the embryo, as one would expect, 
growth takes place chiefly at the distal end of the developing nerve. A nerve once 
formed, no doubt, increases in length, but, probably, only in proportion to the 
erowth of that part of the embryo in which it lies. At the distal end, however, 
growth is much more rapid, and so the nerve wanders out into new territory. 
The frontal and nasal nerves in C.R., F.M., and Mr., offer good examples of 
this. We notice that in passing from one of these stages to the next the frontal or 
nasal nerve may increase by 10 to 20 per cent. of its original length, while the 
increase noted for their terminal branches (supraorbital, supratrochlear, nasal 
proper and infratrochlear) varies from 30 to more than 100 per cent. 
For the embryos C.R. and I’. M. it seems probable that the diagrams exaggerate 
the length of the undivided trunk of the ophthalmic nerve. 
In a series of sections, cut more or less obliquely through a dividing nerve, it 
must necessarily be difficult to determine accurately the precise point at which the 
division takes place. This is the case in both these embryos. In Mr. on the other 
hand the sections cut the ophthalmic nerve at right angles to its direction, and 
the point of division can therefore be determined with precision. This probably 
explains why the ophthalmic trunk appears shorter in Mr., than in C.R. and F.M. 
It is just possible, however, that in Mr. there is a real shortening of the trunk, due 
to a forward growth of the Gasserian ganglion absorbing, as it were, a portion of 
the nerve. Whilst this may be suggested as a possible explanation, there is abso- 
lutely no proof that the Gasserian ganglion shifts its position in this manner. 
The study of the nasal and other branches of the ophthalmic nerve in the 
embryo affords an explanation of the complications observed in certain parts of 
their course in the adult. Professor His* gives an account of how the courses of 
nerves in the embryo are modified by the bending of the tissues in which they lie, 
and by their meeting with obstacles in their growth outwards. Many of the 
factors referred to by Professor His, as determining the course of nerves, are 
illustrated by the growth of the branches of the ophthalmic nerve. Starting with 
embryo Br;., we observe in it a nerve which has a course upwards, and a little 
inwards, behind the eyeball (fig. 5, Plate I.). This nerve, as we have seen, repre- 
sents the nasal of the adult, and its direction corresponds to the direction assumed 
by the processes of the neuroblast cells of the Gasserian ganglion from which the 
fibres of the nasal nerve arise. 
In Rw. the course of the ophthalmic nerve is at first upwards, but the direction 
of the nasal nerve is mainly forwards (fig. 6, Plate I.). 
This change in the direction of the nasal nerve seems to receive explanation 
by a growth forwards of the cerebrum, which, of course, carries with it the 
* «Geschichte des Gehirns sowie der centralen und peripherischen Neryenbahnen beim menschlichen 
Embryo.” Abhandlungen der Konigl. Sachs. Ges. d. Wissensch. 1888, Bd. xiy., p. 386. 
