484 PROF, MILNES MARSHALL AND W. B, SPENCER. 
mate more or less complete fusion of the roots of the fifth and 
seventh is of great interest, as proving that the fusion of these 
two nerves, so characteristic of Pisces and Amphibia, is a purely 
secondary feature, and that the two are at first as independent of 
one another in these forms as they are throughout life in the 
higher Vertebrates. 
In adult Elasmobranchs the combined roots of the fifth and 
seventh nerves are usually described together, and the descrip- 
tions of different observers, though not quite in harmony with 
one another in certain details, yet agree fairly well on the main 
points. Stannius, whose descriptions are the most elaborate, 
describes the combined fifth and seventh nerves as arising in 
Plagiostomes by three roots,! of which one is seen on closer exa- 
mination to be double, giving four roots in all; of these the first, 
or most anterior one, arises from the ventral surface of the 
medulla by two short non-ganglionic roots, which unite together 
shortly after leaving the brain. This root is in Raa, according 
to Stannius, mainly motor, supplying the muscles by which the 
respiratory movements of the anterior wall of the spiracle are 
effected, and also certain others in connection with the jaws. 
The second root of Stannius is large, lies posterior to the first, 
and is in close proximity behind with the auditory nerve ; it 
may be distinguished into an anterior part which belongs to the 
trigeminal, and a posterior, more ventrally situated portion, 
which belongs to the facial. The third root is very large and 
much more dorsally situated than the other; it is connected by 
its deeper fibres with the second root, whilst, from its superficial 
fibres are derived, according to Stannius, the ramus ophthalmicus 
superficialis of the fifth, and also, in part, the maxillary and 
buccal nerves. 
Gegenbaur,” in his account of the cranial nerves of Hexanchus, 
distinguishes between the roots of the fifth and the seventh. 
He describes the fifth as arising by the union of two trunks of 
about equal size, an anterior and a posterior; of these the ante- 
rior (a) arises from the ventral surface of the medulla by two 
roots situated very close together ; the posterior (4) has also two 
distinct roots, a dorsal one (a) arising from the side of the 
medulla by a large swelling projecting into the fourth ventricle, 
and a ventral one (3) situated immediately above the root of the 
facial, and in front of, and above that of the auditory. 
The seventh nerve in Hexanchus is described as arising by two 
1 Stannius, ‘Das peripherische Nervensystem der Fische.’? Rostock, 
1849, pp. 29 and 30. 
2 “Ueber die Kopfnerven von Hexanchus,” ‘Jenaische Zeitschrift,’ 
Ba. vi, 1871, pp. 501, 502, and 5138, 514, 
