378 MR. ST. GEORGE MIVART ON THE VERTEBRATE SKELETON. 
The branchial arches of osseous fishes and of Amphibia appear, then, to me to be most 
probably hypaxial elements belonging to the category B—hypaxial elements because 
they are hard parts either placed within the inner wall of the pleuro-peritoneal cavity or 
serially homologous with parts so placed, and belonging to the category B because they 
embrace or tend to embrace a portion of the alimentary tube. "They are thus considerably 
different in nature from the parts (chevron bones, hypapophyses, &e.) which constitute 
the category A of hypaxial parts. The latter are essentially in close connexion with the 
skeletal axis. "The hypaxial parts of category B, on the other hand, are essentially related 
to the alimentary tube and its prolongations. Thus, when these latter hypaxial parts exist ' 
without corresponding solidifications of the category A, it is evident, à priori, that they 
may be quite detached from the skeletal axis, may arise* independently, and may have 
not only primitively but even ultimately, no solid connexion with it. 
If this reading is correct, the nerves which descend beside these branchial arches (Plate 
LIII. fig. 20, 2) cannot answer to the abdominal branches of the spinal nerves, but are rather 
hypertrophied representatives of the sympathetic. Now the branches of the vagus which 
descend in the posterior mediastinum should also be considered sympathetic, as that space 
is the upper portion of the internal laminæ, 4. e. within the pleuro-peritoneal cavity. 
Moreover Professor Huxley in his last Hunterian course supported the view of the 
essentially sympathetie nature of the vagus; and, as regards the branches supplying the 
branchial arches, he said “it is probable that these are distinct nerves, the true origins of 
which have become quite inconspieuous, while their antero-posterior commissural fibres 
have become exceedingly developed ”+. 
It remains to notice the branchial arches of the Elasmobranchs and of the Lamprey. 
In the Elasmobranchs (Plate LIII. fig. 3) it is manifestly the internal cartilages 
(within the arterial branches) which correspond to the branchial arches of osseous fishes. 
To what, then, do the external cartilages correspond? I should suggest that they are 
paraxial parts, and really do correspond to the ribs and hæmal arches of posterior seg- 
ments of the trunk (fig. 3, P). 
In the Lamprey, on the other hand (Plate LIII. fig. 4), the vessels are internal, and 
therefore the cartilaginous basket cannot at all answer to the branchial arches of osseous 
fishes; it answers only to the external branchial cartilages of the Elasmobranchs, and so 
is probably also paraxial in its nature (fig. 4, P). 
This view is confirmed by its line of suspension, which runs along the side and outer 
wall of the skeletal axis. The branchial arches, on the other hand, are suspended, as 
manifestly in certain Elasmobranchs, tc the under surface of the spinal column (Plate 
LIII. fig. 5). 
The cartilaginous protection of the heart of the Lamprey is thus no part of the hypaxial 
* If the resemblance to Vertebrates lately discovered in larval Ascidians be a resemblance due to genetie affinity, 
and not, as is quite possible, to some similarity of conditions, then this part of the hypaxial system may turn out to 
have originally arisen in a different mode from the rest of it. But the generalized expression of facts here adopted 
will none the less apply to the skeleton as it exists in the five vertebrate classes; as was before said, the outcome 
and result of development must have its weight as well as the mode by which that result is attained. 
t Loc. cit. May 22, 1869, p. 466. 
