HEAD CAVITIES AND NERVES OF ELASMOBRANCHS, 91 
rectus externus v. posterior. This nerve is identical in structure 
and relations with the nerve I have previously identified and 
described in the Chick as the sixth or abducens nerve.) If 
fig. 38 be compared with the figure I have elsewhere given of 
the sixth nerve in the Chick,’ their identity will be recognised at 
once. 
In fig. 39 an enlarged view is given of the nerve from the 
same section as fig. 38, in order to show its structure more 
clearly. The nerve itself is slender; it arises by a number of 
roots, eight or nine in number, arranged antero-posteriorly, so 
that its origin covers a considerable length of the base of the 
brain. Both the nerve itself and its roots consist of fusiform 
or still more elonyated cells. Neither in the nerve nor in any of 
its roots are there any ganglion cells. 
I have recognised the sixth nerve having all the characters 
above described, except a slightly smaller number of roots, in 
embryos of stage M, and also at stage L; in embryos younger 
than t I have failed, after most careful searching, to detect it. 
In figs. 28 to 31 the sixth nerve is shown in transverse 
section at a stage between Mand n. Fig. 28 passes through the 
rectus externus muscle on both sides, but is entirely in front of 
the nerve. Fig. 29, which is taken from a section a short distance 
further back, shows on the left side the sixth nerve cut trans- 
versely ; on the right the posterior end of the rectus externus, 
and a small fragment of the nerve are shown; this section 
passes through the point at which, in fig. 38, the nerve bends 
down and loses itself in the investing mass. ; 
Fig. 30 is taken still further back; it passes through the 
roots of origin of the sixth nerve on both sides. These roots 
lie entirely above the imvesting mass (7. v.), as is shown also in 
fig. 38. In fig. 31 the roots of the nerves (vi) are shown on a 
larger scale from a section belonging to the same series as figs. 
28 to 30; it shows very clearly the non-ganglionic character of 
the roots. 
Figs. 29, 30, 37, and 38, enable us to determine accurately 
the situation of the roots of the sixth as compared with the 
other nerves. Jig. 29 passes through the root of origin of the 
seventh nerve and through the whole length of its palatine 
branch (vit 4.) ; it also passes through the sixth nerves just in 
front of their roots of origin. Fig. 30, which shows the roots 
of the sixth nerves, passes also through the extreme anterior 
ends of the auditory vesicles (avd.), through the main trunks 
of the seventh nerves, and through the auditory nerves (v111) 
before they have reached the auditory vesicles. 
' Loe. cit., p. 32, seg. 
2 Loc. cit., plate ili, fig. 25. 
