HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 83 
proof necessary for the complete establishment of the view in 
favour of which Professor Schwalbe has brought forward such a 
mass of anatomical evidence, viz. that the ciliary ganglion is 
primitively a ganglion belonging to the stem of the third nerve. 
For a full account of the various modifications presented by the 
ciliary nerves in adult Vertebrates I would refer the reader to 
Professor Schwalbe’s very interesting memoir cited above. 
On one point, however, I cannot completely agree with Pro- 
fessor Schwalbe, z.e. when he says that the ciliary ganglion is 
the homologue of a spinal ganglion.t That it corresponds in 
part there can be, I think, no doubt; but it seems to me pre- 
ferable to regard the ganglion at the root of the third, together 
with the ciliary ganglion, and any intermediate ganglia that may 
be found (for which vide Schwalbe’s paper) as collectively 
equivalent to the Gasserian ganglion, or to one of the spinal 
ganglia. I am disposed to view the third nerve as having been 
abnormally pulled out and lengthened by the rapid growth of the 
part of the brain with which it is connected, and to regard the 
whole trunk, from the root of origin to the point of division at 
the ciliary ganglion, as corresponding to the part of the fifth 
bearing the Gasserian ganglion, and to compare any ganglia 
that may occur on this part of the nerve to detached portions of 
the Gasserian ganglion, isolated by the lengthening process which 
the third nerve has undergone. 
In naming and determining the other branches of the third 
nerve, those I have marked 111 @ and 1114, I have been much 
assisted by the very careful description given by Schwalbe of the 
nerve in the adult Seyl/ium. 
Schwalbe? describes and figures the third nerve in the adult 
as giving off branches to the rectus superior and rectus internus, 
then receiving at the ciliary ganglion a branch from the fifth 
nerve, and then dividing behind the posterior border of the 
rectus superior into two branches, of which the first passes 
beneath the rectus inferior, supplying it with branches, and then 
runs forward beneath the optic nerve to the obliquus inferior, 
in which it ends. The second branch runs forward on the inner 
side of the eye, piercing the sclerotic; it passes beneath the 
rectus superior and obliquus superior, but over the optic nerve; 
it leaves the orbit in front by a canal above the origin of the 
obliquus inferior, and then runs forward to the anterior part of 
the head. 
This description and the accompanying figures show that in 
Scyllium the third nerve has acquired by stage x all the principal 
branches of the adult, and that these branches have also acquired 
1 Loc. cit., p. 68. 
* Loc. cit., pp. 15, 16. 
