D1xon—On the Development of the Branches of the Fifth Cranial Nerve in Man. 29 
the other hand, passes outwards below these two branches of the ophthalmic, and 
at once sinks into the external rectus muscle. The fourth nerve which passes 
inwards and downwards, to the outer side of, and a little behind the third, crosses 
over the top of the Gasserian ganglion, and then runs along the upper surface of 
the frontal. During this part of its course the fourth nerve is very closely 
connected with the frontal, it leaves the frontal finally by passing inwards to 
supply the superior oblique muscle. A small communication seems to exist 
between the third nerve and the undivided ophthalmic. 
The nasal nerve is the smaller of the two nerves into which the ophthalmic 
divides. At first the nasal passes forwards beneath the frontal, whilst the trunk 
of the third nerve lies to its inner side. Soon the upper branch of the third nerve 
passes outwards in the angle between nasal and frontal nerves. Before, however, 
this branch of the third has crossed the nasal, the nasal itself has crossed the sixth 
nerve and come into contact with the ciliary ganglion. This ganglion lies below 
the nasal nerve, to the outside of, and behind the optic nerve. The nasal nerve 
when it leaves the ciliary ganglion, crosses the optic nerve lying very close to its 
upper surface. Beyond the optic nerve the nasal has a course downwards, 
forwards and inwards, to the interval between the superior oblique muscle and the 
internal rectus. Passing between these two muscles the nasal enters the interval 
between the vertical cartilage at the side of the nasal fossa, and the cartilage of the 
basis cranii. The nasal nerve now has a course to the outer side of the olfactory 
nerves, and then turns downwards as well as forwards, lying behind the plate 
of cartilage, which connects the mesial vertical plate with the lateral vertical 
lamina, developed in connection with the nasal fossa. The nasal nerve ends 
just above the opening of the nasal fossa on the face. The entire length of the 
nasal is 3°5 mm, 
The infratrochlear branch which arises from the nasal just before the latter 
leaves the orbit is 1°3 mm. in length. 
The frontal gives a small communicating branch which joins the nasal, just 
before the latter enters the orbital canal. Of the two terminal branches of the 
frontal, the supratrochlear passes forwards and downwards towards the surface, 
while the supraorbital passes forwards and upwards. The supraorbital is at least 
3°0 mm. long, while the supratrochlear is about 1-6 mm. in length. 
The frontal nerve passes almost horizontally forwards; it has, however, a 
slight inclination downwards and outwards. At first it lies between the nasal 
nerve below and the fourth nerve above, with the latter of which it communicates. 
In its course it comes to lie on the most posterior and internal part of the superior 
rectus muscle, and here it crosses the upper division of the third nerve. Just at 
this point the lachrymal is seen to take origin from the frontal, and pass horizon- 
tally outwards towards the surface of the head, above and to the outer side of the 
F 2 
