ON THE VERTEBRATE SKELETON. 295 
physial part (51) has, also, retained its slender rib-like form* ; it has coalesced 
with the hemapophysis (sz), and the inverted arch is completed, as in the 
crocodile, by a hzemal spine, as much modified in form by flattening and ex- 
pansion as is the neural spine represented by the supraoccipital (3). The 
diverging appendage of the occipito-hemal arch also retains much of its 
primitive simple character: a long and slender bone (53) supports two rays 
(4, 55), and there is an attempt at three at 57, of which one is short, atrophied 
and anchylosed to the rest. In the two small bones (s6, 56) interposed be- 
tween this and the preceding segment, we recognise the special homologues 
of the carpal series in the crocodile and fish: in 51 we have the ulna, in 55 
the radius, in 53 the humerus, in 57 the metacarpus ; in d 3 and da the rudi- 
ments of the digits so numbered in the crocodile (fig. 22) and the mammal 
(fig. 24). The evidences of the unity of plan in the construction of the 
scapular limb, whether it be an arm with the prehensile hand, a hoofed fore- 
leg, a wing, or a fin, are admitted by all; the same scapula, humerus, anti- 
brachial, carpal, metacarpal and phalangial bones are readily recognised by the 
tyro in comparative osteology in the ape, the horse, the whale, the bird, the 
tortoise and the crocodile. The beautiful simplicity of the fundamental basis 
of all these adaptations of structure is descanted upon in all our popular 
teleological treatises. But the higher law governing the existence of these 
special homologies has attracted little attention in this country. Yet the 
inquiry into that more general principle of conformity to type according to 
which it has pleased the Creator of organic forms to restrict the manifesta- 
tions of the variety of proportion and shape and substance and even relative 
position of the limbs requisite for the various tasks assigned to the vertebrate 
species, is one that by no means transcends the scope of the comparative 
anatomist. And the conclusion to which my comparisons have conducted 
me is, that one and the same element, viz. the diverging appendage of the 
occipital vertebra, forms the seat or substratum of all the adaptive modifica- 
tions of the’ part called ‘anterior’ or ‘ superior extremity.’ 
The second segment of the skull has for its central element a bone (fig. 
23, 5), which in the bird, as in other ovipara, is connate with that (9) which 
stands in the same relation to the third cranial segment; the proof of the 
natural distinction of these segments is given by the neural, N 11, N 111, 
and hemal, H 11, H 111, arches. Probably the circumstance of the bodies 
of those vertebré being formed by ossifications of the fibrous capsule of the 
‘notochord, representing the external or cortical parts only of such centrums, 
may be the condition, or a favourable physical cause of such connation. 
The neural arch of the parietal vertebra retains the same characters which 
it first manifested in fishes. Besides the neuwrapophyses (6) impressed by the 
mesencephalic ganglia and transmitting the trigeminal nerves, besides the 
vastly expanded and again, asin fishes, divided neural spine (7), the parapo- 
physis (a) is independently developed. It is of large proportional size ; and, 
owing to the raised dome of the neural arch, is relatively lower in position 
than in the crocodile; it sends downwards and outwards an unusually 
long ‘mastoid’ process, and forms a large proportion of the outer wall of 
the chamber of the internal ear with the bony capsule of which it speedily 
coalesces. : 
The hzmal arch of the parietal vertebra (H 11) is more reduced than in 
the crocodile, and owes much of its apparently typical character to the re- 
tention of the thyrohyals (46, 47) borrowed from the branchial arches of the 
* The very common modification of form which this element undergoes in becoming ex- 
panded into the broad scapula of man and other mammalia, appears to have influenced Oken 
in his idea of that bone being the homologue of a congeries of ribs, 
