ON THE VERTEBRATE SKELETON. 295 



physial part (51) has, also, retained its slender rib-like form* ; it has coalesced 

 with the hcemapophysis (52), and the inverted arch is completed, as in the 

 crocodile, by a haemal 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-haemal 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 (56, 56) interposed be- 

 tween this and the preceding segment, we recognise the special homologues 

 of the carpal series in the crocodile and fish : in 54 we have the ulna, in 55 

 the radius, in 53 the humerus, in 57 the metacarpus ; in d 3 and d 4 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 in, 

 and haamal, H n, H in, arches. Probably the circumstance of the bodies 

 of those vertebrae 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 neurapophyses (e) impressed by the 

 mesencephalic ganglia and transmitting the trigeminal nerves, besides the 

 vastly expanded and again, as in fishes, divided neural spine (7), the parapo- 

 physis (s) 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 haemal arch of the parietal vertebra (H n) 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. 



